Firstly do not rush through things. Both of you evaluate your commitment and the love you have for each other. He's an excise inspector, you are preparing for a government job. You are almost there and if the bond between you two is sincere and unshakable, you will have a good life together.
The boy's father is a heart patient, no son or daughter would like to upset or bring a set of worries to such a father. Please take some time out, have a frank and sincere discussion that is realistic and not overtaken by emotion. What is the strength between you two, can either of you wait for 3 months and give your love a chance to succeed. Do you have the will power to stay apart and try to make it work. Is the love between you two heart felt or based on a steady job or circumstances of family wealth or being acquintances.
Usually, it would be the other way around, the girl's parents would be difficult to be convinced. You have managed to convince your family, now he has to and this will happen with time. You need to find out what is the core issue that the boy's father is objecting about, you not having a job or a internal hiearchy in the sub-caste that would make him feel that he is losing face.
See whether the boy has a sister or a sister in law. You can try and make efforts to get to know the women in the family while you are preparing for the exams. Once you give them a picture of confidence and maturity that you both are willing to wait, they will slowly start accepting you. This would also enable the boy's father to hear about the intentions of you both.
After some time, when the time is right you can broach the subject along with elders from both families so that the boy's father agrees for it. Now, your fiancee should be assertive and show his father that you both are sincere and would be a good couple without being a burden on anyone and his parents would be looked after well by you both.
If all goes well fine, if not, the boy and yourself need to take a decision and get together, with time the families would bond. If you have little time for your exams, finish the exams, get a Government job and see if this changes the attitude of the boy's father.