Doing Science Graduation after Mech. Engineering

As far as admission criteria are concerned there may not be any problem for a candidate to get into B.Sc Science stream after doing BE/BTech in Mechanical Engineering. BE/ B. Tech is a 4 years course. That means he has already spent 4 years after Intermediate. Now if he wants to join in B.Sc which is a 3 years course, he has to spend another 3 years in studies only and ultimately he will be left with two graduate degrees. Both the degrees are nor related and the jobs will also be different. So one has to think about spending another three years for this.
I have a suggestion. He can get into any job and do B.Sc in distance mode. He can take admission in any of the open universities or correspondence schools and continue his studies. He can opt for the subjects he wanted and do the graduation in Science.
There is another option for him. Institution of Chemists ( India) is offering a course called AIC which is equivalent to M.Sc Chemistry. This is for working professionals. Your son after joining in a job can try for this course also. That will give him a PG qualification.
Another way is to do AMIIChE. This course is equivalent to B.Tech in Chemical Engineering and the course is offered by Indian Institute of Chemical Engineers. This is also a part-time course for working professionals. As he is already passed mechanical engineering there may be exemptions for some papers.
So going for a correspondence course is the best solution for this. He can try any one of the above options.
Annamalai University, Indira Gandhi National Open University and many more Universities are having B.Sc courses.

Since your son is in the final year of Mechanical Engineering and once he acquires this qualification, he should make his endeavour to get a job in his related area without loosing his time in some non productive assignments. To make the use of time, he needs a further revision of the subjects already taught in his syllabus especially the fundamentals related to different boilers, efficient utilisation of steams, procedure of mechanical maintenance of work - shops etc. He has to be effective in his communication skill so as to grab a post in his area. Earlier, he starts a career, he would enjoy better promotional prospects apart from better pay package for the continuity of service.
In regard to his admission to B.Sc course, it would not be difficult in the stream he likes. His admission would be considered on the basis of his marks secured in the intermediate stage. Doing this course from a regular college would mean that he should defer the line of action in his well thought planning and the time consumed in this course to the extent of three year cannot be compensated later. The best way would be to take up this course from the distance education mode from any one of the universities such as IGNOU, Karnataka Open University, Annamalai University etc where your son would utilise his leisure hours for the preparation of the examination.
Now I would suggest some fruitful courses which will help in his career advancement. These are as follows-
1) AMIE in Chemical Engineering - At the starting point, you will be waived from the Section A containing eight papers by virtue of your qualification in Mechanical Engineering. In Section B, there will be altogether nine papers and you may clear the papers as per your convenience taking them part wise in a given session. In a year, you will have two sessions to write your papers. To expedite the clearance of papers, you may enrol in an established institution for better guidance.Thus you will have dual qualification both in Mechanical and Chemical Enginnering accelarating your promotional scopes in your service career.
2) Advanced Diploma in Industrial Engineering- You may procure this diploma from a recognised institution where you will be taught how much time should be devoted to carry out the single operation of the plant. This course would dwell on effective utilisation of man - power, curtailing the maintenance hours of shops, studying various models of operations where quality production is achievable with the less usage of some components thus saving our production cost.
3) Take Management degree through the distance mode - This could be in the area of Production, Finance or Management of computers. Proficiency in one of areas will help your son to have accelerated promotion.

I have seen many people who completed their engineering and after that do not want to study more sometimes because they have already spend 4 years in engineering for a graduation degree . But your son like biology so he wants to do BSc, its not seems too good idea. There are many more ways to follow you passions so if he wants to go with science stream so he should choose a diploma course related with biology , and at the same start a job and should not waste these valuable days of life. There are some diploma courses available -

Diploma in Science: Atoms, Prescriptions, and Medications
The course uses some thoughts from science to build up a portion of the aptitude of the aspirants. These are related to the reality at the atomic level. Substance thoughts are clarified when required. If you are totally new to the dialect of science, then you will need to have some patience while you get comfortable with the vocabulary.
Diploma in Bio science for Worldwide Sickness
This course is to educate you on ongoing advances in the field of Bio science. You will thus understand the effect of Bio science on overall well being globally.
Nutrition and Dietitian
As illness due to lifestyle has increased, there is a huge demand for the dietitians and nutritionist. One can easily find a job in- health and fitness clubs.
Diploma In Physiotherapy
The Physiotherapist treats disease & injury by some physical treatment without medicine. They treat disease or injury through Massage, exercise, heat treatment etc. Patients of orthopedic, cardiology etc. need physiotherapists for their quick recovery. Gyms, Sports centers also need Physiotherapist.

There are many more option to pursue science, BSc will take tree years which is a long time so I would suggest please go with any of science diploma courses.
Still if your son does not want to do diploma and only ant to Bsc then ask him for a correspondence course of BSc. There are many universities which provide the facility of distance learning study.
Some of them are -
Madhya Pradesh Bhoj (Open) University. ...
Dr BR Ambedkar Open University (BRAOU) ...
Netaji Subhas Open University.
IGNOU. ...
Acharya Nagarjuna University (CDE) and its Features
Symbiosis Centre for Distance Learning and its Features
Sikkim Manipal University and its Features

If someone has a passion for pure sciences then definitely one can join the science graduation course after doing the engineering also. But we have to see the other aspects also in the matter. First thing is in regular courses generally the upper age limit for getting admission in B.Sc. is around 23 years and that point is to be considered in this case. Of course option of doing it from correspondence courses is very much open where there is no such age limit.

In my view Mechanical Engineering itself is a science subject and almost all the basic principles of science are used there. Even then one can pursue one's aspirations by doing some distant education course in pure science for which we have so many institutions available and moreover it can be done on a part time basis not wasting the valuable time at this juncture of making career. If there is an aspiration for doing research in pure science then one will have to go for a PG degree also in succession and then try for registration for PhD. It is very much possible though it is a round about route. There are many mechanical engineers who after completing engineering degrees, joined some industry where they encountered some scientific work and had learned a great deal. It is not that one acquires knowledge only during education. If one has interest and real passion the whole life is meant for it.

So, it is advisable to quench the thirst for pure science through a parallel distant education path and continue pushing Mechanical Engineering stream as a career. Just to encourage and motivate, I want to quote one example here. Long back, one of my friends who did his M.Sc. in Electronics, joined an upstream petroleum oil and gas company and within 5-7 years he was working like a Geologist and was made a coordinator for all the upstream exploration activities. We asked him the secret of his success. His answer was that he read the basic Geology through self study and did some part time course in it and found that it was only a fundamental science and no way different from other learnings. So, one can add any learning at any stage and should not sacrifice one's career just because one has a passion for something back in time. I would not suggest someone for not making a career in his core area and going back to the rudimentary knowledge.

New generation people have many inspiration. There have been seen many people in society as your son. They want to pursue a different stream. Since your son is going to complete a B.tech degree and already he spent more than three years in taking degree so he should not waste time and energy. And at first, he should join a job in his technical field. After that, he should apply for B.sc(biology). While doing the job it is not possible for your son joining the B.sc course in regular mode. So, you can do it through distant mode. There are many open universities in India which provide bachelor degree in distant mode are as follows:-
1. Indira Gandhi national open university
2. Karnataka open university
3.Annamalai university
4.Acharya Nagarjuna University.
Apart from these, if you having high inspiration for pure science then there are different diploma course available by which you can fulfil your dream. A diploma course would take less time than a degree course. Some diploma courses are considered as best from a career point of view such as a Diploma in physiotherapy, a diploma in bioscience etc.