My kid brother used to hang out with an older crowd. He wound up becoming quite rebellious during this time and sneaked out of the house often, not returning for days at a time. He picked up cigarette addiction, drinking, and who knew what else. My parents had him tested once or twice for illegal drug usage, because he wouldn't really be to open to communication during this time.
Not saying this is what's happening to your son, but I'd be weary all the same.