I have no idea if this will help, but:
I was a gifted and talented child; my grandmother was a schoolteacher. I was a real brat and I ended up leaving school early anyway, mostly because there was never any challenge or support.
Anyway, my grandmother taught me to knit (I think she did this with her bright students, too) and then made me knit whilst I was in school, during the lessons -- it slowed me down enough to think about two things that I didn't act out.
Probably finding anything to do with his hands while he simultaneously does his lessons could help him calm down in class. Just make sure his teacher knows what's going on.