Wow, Can you please elaborate on what kind of extra-curricular activities he has already done?
I would think that any boy who tries karate would feel empowered by it and would want to continue with it until he gets his black belt (and perhaps beyond that as well) Karate teaches self-discipline/respect which he seems to be lacking at the moment… it would also provide him with a healthy alternative to relieve his stress/anger while simultaneously helping his self-esteem issues.
also, perhaps he is more artistic and would want to take art classes? or woodworking classes?
Perhaps he likes to cook and would enjoy taking culinary/pastry classes?
perhaps he is musically inclined and would be interested in learning how to play an instrument or two and start/join a band?
perhaps he enjoys swimming and would be interested in becoming a lifeguard?
perhaps he enjoys working with cars and can find an auto mechanic shop where he can be an apprentice and learn all about cars?
perhaps he likes computers and would benefit from a computer programming class?
You can find several different programs/classes (like the ones listed above) that you would approve of and just hand him the brochures and let him decide if he wants to take any of the classes or not. dont pressure or force him into it, like you said-the decision to attend these classes has to come from him. its really up to you to show him that there are tons of things that he can do and that you would be willing to support him if he chooses to attend any of the classes. You never know if he will take you up on the offer or not but you can at least try.
Good Luck!!