I'll make my usual point: if they're going to need special treatment at school, it's a lot easier if they *obviously* need special treatment at school.
Amen to that! I only WISH we'd understood what you do at that age! Kudos to you for recognizing it early and providing the challenge. It sounds horrible, but I'm afraid my son will never realize his potential because we allowed him to under-achieve for so many years. I think your son sounds amazing, and I completely agree that as long as he's enjoying it, allow him to keep going. If he shows any signs that he's not enjoying the "work" , give him the freedom to back off. I think providing challenge without forcing the issue is a good position to take on "how much is too much".
I didn't get a chance to read all the responses, so hopefully I'm not being redundant!
