S.B.
Dear T.,
My daughter never took a pacifier until I took her bottle away. She never sucked her thumb until I took the pacifier away. Thumb sucking was a comfort thing for her. My husband hated it and always threatened to put hot sauce and horrible tasting stuff on her thumbs to MAKE her stop, which just made her suck her thumb more. He was a control freak and hounded her constantly about it which helped absolutely nothing. You can't take a kid's thumb away from them all together.
My daughter and I made a deal. I would not give her a bad time about sucking her thumb, but she was to do it in private. She was still sucking her thumb in the third grade and at slumber parties, she would crawl down inside her sleeping bag to suck her thumb so no one would know.
They really do eventually grow out of it. But I think the more you hound them, the worse it gets. Tell him he can suck his thumb, but not at school, not in the car, not in public, etc. If we were watching a family movie and my daughter covered herself up with her blanket, we knew exactly what she was doing, but left her alone because within 5 minutes she was asleep.
They get to the age where they don't want anyone to know they suck their thumbs or be teased by their friends about it. They do quit on their own. Trust me.