My son is grinding his teeth but he is doing it while he is asleep. How do I get him to stop if he is sleeping?

My son is 17 months and the sound of his teeth grinding is keeping us up at night and make me sick to my stomach each time he does it. I have tried to turn his head from one side to the other without waking him because he needs all the sleep he can get but it doesn't seem to help. He will stop for a few minutes then start again. It seems the only thing that works is giving him a pacifier or bottle but we are trying to phase out both. He does not do this when he is awake so trying to give him things to take is mind off of it or tapping his teeth is not a solution for us. What should I do?