Dear K.,
I agree with the other moms, that he could be copying another child about the thumb sucking. I do know that thumb sucking is a way that children self sooth. I started sucking my thumb in the infant stage and my sister sucked her finger. I think that we started because our parents never came in to comfort us at night when we cried so we had to learn another why to get comfort. You should try to do a little investigating to see if something is bothering him. I also sucked my thumb until I was 13. WOW, I know that's what you must be thinking, but my parents ignored the doctors recommendation which was to ignore it. My parents feared it would cause a really bad over bite, which it didn't. The doctor assured them that if left alone I would eventually stop on my own. After my parents thought I stopped, and they left me alone. I quit with in the next few months. Hope I helped!