I'm no expert, but thumb sucking is a self soothing method, is it possible something is bothering him? Maybe when you see him do it next pick him up and hold him, maybe even ask him if he is sad. If he is using it to soothe and being punished that would explain his lashing out when you try to put him in time out, he doesn't exactly have the best communication skills yet to know how to explain to you how he feels. If he's not using it to soothe then is it possible another kid he's around is doing it? He may have picked it up and thought it was normal. Either way I don't think thumb sucking is something to punish him over, sucking his thumb isn't being a bad boy. My son was always very sensitive, if he started acting differently or lashing out I'd bring him in for a hug and ask him questions to help him verbalize how he felt. He's 11 now and still comes to me to talk when something bothers him. I'm still surprised sometimes at how sensitive a boy can be.