What do you do to help a kid stop thumb sucking?

As a former thumb sucker from waaaaay back I can tell you that it's a habit so it's hard to break. Nagging your son won't work but you could try a combination of rewards and not letting him get too tired and/or stressed. As I recall, being tired or under stress made it worse. So, you could set goals for your son and offer rewards if he, for instance, doesn't suck his thumb for two days in a row. After awhile increase it to three days in a row. Make the rewards small or let them be something that can add up to something bigger. Good luck and, remember, he won't be sucking his thumb when he graduates from high school and his teeth will not be completely ruined by it.