L.,
As a mother, I understand the worry of social stigma and orthodontia prolonged thumb sucking carries...but I hope I can offer a bit of advice:
While I have horrifically been exposed to middle schoolers the suck on thumbs, bottles, & pacifiers, I don't think it caused them any negative social stigma (as it was considered "cool" ....let me take a moment to shudder) and the pediatric orthodontist had already introduced braces in most cases.
I say this as a contrast to the 6 yr old ideal and intent of thumb sucking, as well as to give voice to your worst fear in ragard to this.
Personally, I sucked my thumb at night until I was 12 - every night until about 10, then every coupla of nights. Once I got braces it got a bit painful as the metal would dig into my skin, but I got used to it. Understand, my parents tried everything from duct-taping a sock over my hand, to the bitter-tasting stuff that you paint onto your thumb, to "reward" parties when I could quit for a week or two (that started about 7). Ultimately, tho, it didn't stop until I wanted to. I got through the pain of removinng duct tape from my arm, got used to the nasty bitter thumb stuff so much so that I actually liked it, and would quit just long enough to get a party...but nothing worked until I decided it was enough.
I wouldn't worry to much about it. Yes, it could cause him to need braces later, but really, I don't remember a single kid in school that didn't have braces at SOME point (assuming their parents could afford it). Absolutely encourage him to give it up and attempt to help him by whatever means possible, but understand that unless HE WANTS TO, any measures you take probably won't make much impact.
Hope that helped a little....
Good luck!