S.A.
I don't know if this will apply to you, but I know as soon as my son went to school, he stopped dead cold. I think it was between the social aspect, and the fact that his nail started to crack that freaked him out about it. It definitely made him want to stop doing it. If you know of things that he is concerned about about his thumb, maybe you want to point those things out to him? I don't know, just a thought. Sometimes, I think it's better when we let the kids do things rather than trying to force them to do the things we think they should do. It certainly seems to take much better. Hope that helps a little, sorry if it doesn't.