L.M.
Welllll, I'm biased. I sucked my thumb until the 5th grade! In private, of course, but I never needed braces and there was no teeth damage. I say let him do it, as long as it's not causing any social problems for him. If he's doing it in school and being teased, then you may need to talk to him about appropriate times and places. One day he will stop, on his own. I don't think there is any need for either of you to stress about it :)