I'd get him checked out by your dentist soon to see if ther are any problems developing. I have ground my teeth for as long as I can remember, and could crack my jaw on both sides for many years before my parents thought to do anything about it! I now wear a plastic night guard on my upper teeth which is great - no jaw pain, no worn down teeth. I got mine at the age of about 12 though, so I don't know how they deal with baby teeth. Your dentist should be able to advise though.
My 4 year old son is currently fast asleep grinding his teeth. He's never one it before and the noise is making me feel a bit queezy in that I hope he's ok kind of a way. Is it ok? Should I be doing something to stop it, if so, what? Thanks