beau, I'm in HR and have probably interviewed well over a thousand individuals over the years. I can't tell you how many times I've heard that "It wasn't me; it was the people I was around" excuse when a candidate has tested positive for marijuana. (I don't know a single HR person who hasn't heard that excuse dozens of times either.) Unless your son was confined in a room with no ventillation for days with a group of people chain smoking dope, he's lying. Nobody tests positive for marijuana unless they've been smoking it themselves.
I'm not giving you a smart-ass answer. The sooner you accept that your son is using drugs and get him into some sort of treatement program, the better off he will be. If you elect to believe this nonsense he's dishing out, you're just enabling his continuing drug use.
Good luck.