C.C.
Honestly, there is nothing you could do to get him to stop because it's involuntary. Our son is 4 and does it when he's asleep. I asked our dentist about it and he looked at his teeth and didn't seem to be concerned. Certainly consider getting him into the dentist to get him adjusted to seeing a dentist while he's young. If you do decide to look for a dentist, find one that doesn't sedate children and works more with short visits to get the child comfortable with having dental check-ups. I avoided looking for a dentist because I thought they all sedated kids and I don't agree with that type of approach but when I spoke to our family dentist he assured me that they also didn't agree with sedation and not all dentists sedate. To this day our son loves going to the dentist and has been so cooperative and I doubt he would feel the same way if we had taken him to one that sedated him.