If it not just a habit and ignoring it doesn't work, he may just have a need to do it. I know that sounds weird, but some kids have some oral sensory issues and have a craving for that type of pressure in their mouths. Does he chew on toys or sippy cups?
What may help is getting that need filled in a less irritating way. when he does start you can just hand him a bagel, gummy bears or anything else you can think of that he may like that is hard to chew. See if that helps and he grinds less. If he continues to do it I would ask you dr or dentist what you can do because he can actually wear the enamel off his teeth and cause chipping or cavities.