Bev is right, summer school is for students who didn't pass a subject. Many kids go to summer camps with specialized focus, like a two week computer camp.

Many school systems offer programs that allow their students to take some of their classes at a local community college. That would give you another option. Where I live in Florida, you can spend your entire senior year taking college courses, and still do all your high school things like play football or work on the yearbook and all that.

You do have to consider the possiblity that you will return to Europe, and your son's US high school diploma may not count for much. I met someone who's boyfriend decided to go live with his Swiss father and go to a Swiss university. When he got there he found that they did not accept his high school diploma and he had to do it all over again. He thought he could rush right through, but it was much harder than in the US and took him two years. He probably would have been better off with the IB.