I have a 3 yr old son, and he is a thumb sucker. And, I was as well when I was a child. We don't bother him about it, because, it is a comfort type thing. He does'nt do it at pre-school, and it is, as with your son, an evening type of behavior, for the most part. I know that it may seem a bother to you, and other people, especially strangers, feel a need to comment on it. But, it is a child's way of coping with stress, and relaxing themselves. I wouldn't worry too much about it, kids are, even at a very young age now, expected to display a level of behavior that is beyond their developmental age. We, as parents, should help them realize, it's okay to be frustrated, scared, angry, perplexed, and out of sorts.