^ You are right, I completely forgot AP English Literature after AP English Language. So, there are four subjects required for graduation as I thought I heard the counselor say (but I could not remember what the fourth subject was).
In any case, thanks for helping me see that it is quite common in US schools, for kids to take more courses than they need to graduate. I think there is a consensus that instead of taking four classes and skipping the last three periods, he should stay and atleast take AP Physics and AP Calculus. I will look into dual enrollment to see if he can take an interesting third course too. Even so, it appears he will have a very light senior year compared to his junior year.