J.P.
My son used to do this often while he slept. It has gotten much better now. (He is 5 now.) I don't know if there is much that you can do. I would agree not to bother waking him up because he can't control that action once he is a sleep again. Check with your dentist and see if she/he has any advice. I used to think with my son.... well at least it's his primary teeth. Hopefully your son will not grind his teeth as often as he gets older.