Hi. My son did it as well, and it really bothered me. He started at about 2 and did it until kindergarten. I tried to discourage it by telling him it would hurt his teeth. He tried (it was never turned in to a power struggle issue.) But he even did it in his sleep. Our dentist said a lot of little ones do it, but when their baby teeth come out and new ones grow in (usually kindergarten age) they simply stop. It feels funny to them. And sure enough, the wobbly teeth were no fun to grind and the new ones were too bumpy.
I'd ask your dentist to be sure, his case may be different. We also did get hot and cold sensitivity because the nerves were closer to the surface from all the grinding.
I just thought you'd like to know that it's not too uncommon, and not worth fighting with him about. Go see your dentist :).
B.