T.L.
He will stop when he is ready. My brother sucked his thumb when he was little too. He is now 32 and when he is sleeping his thumb still goes up by his mouth. However, when he was in high school he cut half of it off in shop class.
If I had to guess it is a comfort thing. Maybe you can find something else for him like a blankie. If he doesn't do it in public and hides when he does it then I really wouldn't think it was a big deal.