First of all please let me know what is your son doing right now? Is he a graduate or an undergraduate? If he an interest in law, then ACS is a very good option.
To register for the course you need to visit your nearest CS chapter to fill up the application form and payment of fees. Some documents like graduation passing certificate or class 10 and 12 passing certificates will be required to be submitted. Where does your son reside? There are different local chapters in various towns and cities. Just check on the internet the address of the nearest chapter and call them to know about the current fees for registration. The study materials will be given to him as soon as he is registered with the Institute.
If your son is an undergraduate, then he has to complete three stages, that is Foundation, Intermediate and Final. If he is a graduate, then he has to complete Intermediate and Final. The names of these stages may be different currently, however the course content and the structure are almost the same.
To complete one stage, he should secure atleast 40 marks in each subject and get a atleast 50% of all the subjects put together. For example: If he gives 4 papers, then he has to secure 40 marks in each subject and atleast 200 marks out of the total of 400 marks of all the subjects put together. If any of this condition is not fulfilled, then all the subjects have to be given again, that is if either he gets less that 40 marks in one subject or gets a total which is less that 50% of the total marks. There are a limited number of attempts to pass Foundation. After passing Foundation, a registration card is given which specifies the date within which you need to pass Intermediate and Final and if you don't pass by that date, then you need to renew registration.
At intermediate and final stages, the subjects are divided into 2 or 3 groups and you can either give one group and can give all the groups together depending on one's choice.
ACS covers all the topics on law, like Company law, Property law, Partnership law, Constitutional law, Information Technology law, Contract law etc.
In addition to giving exams, a compulsory articleship of about 15 months have to be completed under a practicing CS or in a legal services company. There are also other formalities which he will get to know on registration like training at Registrar of Companies, some class room sessions at the Institute etc.