T.T.
My 6 1/2 year old does the same thing and has for years (started when he was 2). For him it's a stress induced coping mechanism. He may go for months without doing it, but will start again when stressed (such as starting school again). I would guess it's the same for your son.
Keep in mind that this is a COPING mechanism and doesn't need to be stopped just redirected. Finding a better outlet would be more productive than just trying to get him to stop. For our son jumping on the trampoline is a great stress reducer. We also give him old wash cloths to chew on when he really feels the need (he'll ask for one). I've got a lot of good ideas from a gentleman named Kirk Martin. He came to our school one time and gave lots of ideas for helping kids. Getting on his email list is free, so you might give him a try.
Good Luck!!