I imagine you are already talking to him about it. You need to because he is going to be in for a lot of teasing in kindergarten. You ought to warn him about the teasing. And when it happens, as it almost surely will, don’t support the critics and tell your son that they are mean. Instead, remind him that we can’t control what others think and say, and help him develop strategies for breaking his habit. For example, tucking his thumb into his fist and holding it at his side, sticking his hands in his pockets. I would think you don’t want to encourage another bad habit for the thumbsucking (such as chewing gum). Finally, don’t worry, he will break the habit. How many grown men or 15 year old boys have you ever seen sucking their thumb? (The issue is not whether he will break the habit, but rather helping him to do so with the least amount of embarassment and the most self-esteem.)