My kids did this too. It is partly because they have discovered something new they can do and partly because it probably does bother him a little if there are more teeth coming through.
Like someone else said, I just gave them something to chew on. They put everything in their mouths anyway so that is not too hard to get them chewing on something instead of grinding their teeth.
I definetly did not try to make them stop. They quit on their own. It was just a new novel thing at the time. Be ready, because it is going to be like that--they keep discovering new things they can do like take their diaper/clothes/shoes off, paint with their food, and other fun things. LOL I never encouraged or accepted any of it, just tried to move them past it by showing them another 'neat trick'. ;-)
Good luck,
ts