He sounds bored.
Instead of focusing on inward reflection concerning outward behavior (i.e., group therapy), why not give him an intellectual challenge? Can he take AP classes? Can he take on more responsibilities, like maybe tutoring other students? Does your local college offer a high school-to-college transition program? Sometimes colleges will allow gifted high school students to take a college-level course in addition to their high school curriculum. You might make taking a college class the reward for staying in school.
I know, staying in school to go to school probably doesn't make much sense. Think of it like this. When I was a kid, I did so poorly in school that I am still astounded they let me graduate. I saw high school as doing time. I showed up, read loads of fiction in class, and passed exams without even having heard the lectures. Because I did so poorly--I mean I brought home F's--anything that might have challenged me was withheld from me. By junior college, I still had this bad concept of myself as someone who wasn't very smart. When I returned to college as an adult I found it ridiculously easy--graduated suma cum laude. It wasn't until my second master's degree that I started thinking, uh-oh, this isn't so easy anymore! It may help your son to know that while lower-level education focuses on knowledge acquisition, upper-level education changes gears and starts asking, okay, now that you know it, what are you going to do about it? It may also help him to understand that diplomas open doors.
The point is, smart kids need challenges. If you want him to stay in school, then challenge him.