Many teens just don't like activities. I was one of them. I wasn't athletic, had outgrown dancing school, and wasn't interested in "thing-centered" clubs. I preferred working part-time and being with the "additional set" of friends at work, and then my few closest friends from school. Although most little kids love activities (I did), a lot of teens just feel they've outgrown them; or else that there aren't any that interest them.
Some kids don't like competing or being in large groups. They prefer closer relationships with a few friends. Some kids just like to find their own interests, rather than feel as if they're "forced". Sometimes, if a kid has his own interests that he really values, he wouldn't want to "ruin things" by "joining a club" centered around what he feels is "his own, special, thing".
There's a difference between "doing nothing" and "not participating in activities". I'd just make sure my son had lots of things to do on his own that were constructive. Having things like skis, a bike, exercise equipment, interesting books, or something to collect or work on that DOES interest him can help keep a teen who doesn't work part-time from being bored. If he's got a couple of close friends and enjoys his time with them, I wouldn't worry about it.