M.R.
I would have him map out his summer break, give him a few days to accomplish this task. This way he can pick and choose. If he is like a lot of teens out there who just wanna watch TV then get the cummunity summer activities newsletter they have something for all ages + you can have input then if he is not proactive about having a plan.
I would do something like this:
Writing class (english) Maybe a couple mornings a week, then have him involved in a sport in afternoons or vice versa. Maybe guitar lessons if he wants to learn music.
Good luck