S.W.
Why don't you try to "bribe" him w/something he really wants. You and him set up a goal on when he will stop sucking his thumb by a certain date, and for every day he goes w/out doing it, give a sticker on a calendar until he reached so many days w/out doing it and then get him whatever you both agreed he would get when he reached that goal.. You could also make it a huge deal when you see him correct himself. If all else fails, you could show him online pic's of crooked teeth and talk to him about what it is doing to his mouth. He's 6 after all, he should bee able to understand. For the record, I was a finger sucker(my pointer and middle finger) and I stopped around 5. It took until my mom had my brother, then I didn't want to be like the baby, maybe pushing the big boy issue will help. But try not to make him feel terrible about himself, it's such a fine line w/kids. Good Luck!