Any chance that he has recently been playing a lot of video games since school got out? That can cause thumb problems.
Yes, he has been playing the games. Its almost like he is playing even though he isn't. I tried to hold his hand to see how long he could keep it still and I could see/feel the pinky and ring finger moving.
I think its possible the games could be a cause - an orthopedic doctor might be able to help you know for sure.
Try some type of an anti-inflammatory med like Advil. Limit his times on the games for awhile. Take a look at how he is holding the game controls. Good Luck!