I know nothing about GCSE / A level. I just wonder that
1. Is it possible if I want to put DS into GCSE exam before his actual age, say 12/13/14?
2. If possible, how can I do that?
3. If he got C grade or lower, can he take the exam again (and again) to get A*? Does the previous grade count?
1. Yes, definitely
2. That depends. If your DS is at a school that offers the qualification (or possibly even just the board), you may be able to have your DS sit the exam as one of the school's own candidates. Check with the examinations officer at the school. If the school won't help, then you will need to check whether the qualification allows for private candidates. (Google the qualification and 'private candidates'). It can be difficult to enter a child as a private candidate, as then you will have to find an examination centre for them. I don't know what would happen if your DS then happened to have a clash with some other external exam his school had entered him for.
3. As far as I know, you can resit virtually anything. However, I know it is difficult (impossible?) to hide previous grades, and depending on circumstances, an earlier poor grade *may* create a problem for your DS. You'd need to check the details out with the examining board, or possibly the examinations officer at your DS's school.