If you just drop him off at a regular gym he won't know what to do. When teenagers are left to their own devices, they tend to do what is comfrtable (curls, bench, tricep extensions etc...stuff to make your arms look bigger) Most big box gyms have personal trainers, but you have to ask a lot of questions before you jump into training sessions because many of those folks have no idea what they are doing either.

Check out CrossFit - those classes will wear him out. They are usually about 1 hour long and programmed and led by someone who knows what they are doing. This kind of high intensity training will get him out of his comfort zone and make him a better athlete overall. Many of those gyms offer student discounts.

Also, you might want to consider removing processed sugar from this kids diet. Do a little research on the correlation between behavior and diet quality. It is interesting.