Hey Plantman,
I'm coming in on this late too but I think I can offer some advice from experience.
I started out smoking pot in High School (cigarettes too). Mostly from peer pressure, maybe a little lack of self esteem too. That quickly graduated to other stuff, mainly alcohol at first. Then other drugs. I'm not going to name them all but I did a lot. I wound up getting help and getting into recovery. But I still have battle scars from those days namely a lost college education, failed marriage, lack of social skills. But the people who didn't make it into recovery 90% of them are dead. Yes you read it right DEAD, as in buried in the ground.
If you think this is going to stop with pot your seriously mistaken. And I agree that he should be professionally evaluated but it sounds to me like he's playing you like a fiddle. I know, I did the exact same thing with my parents. YOU are the adult and the parent. You should not be wworried about hurting his feelings. He needs guidance with love from you. You can worry about being friends later.
YOu need to know where your son is at all times. You also need to know who he's with and what he's doing. If this means he has to check in every hour or two so be it. If he says he can't get him a cell phone.
I have a friend who's son checks in with her every hour by phone when he's out at night. If he doesn't call she does. I found this to be kind of extreme but she says all the mothers do it. And if he doesn't he's in trouble. My favorite punishment is removing the bedroom door.
My advice to you would be "Get on him!" and don't let up. Don't take the "I'm going to sit back and see what happens" attitude. You already tried that, it doesn't work.
I wish you luck, it's not going to be easy.

Danny

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