I'm going to get a lot of shit for this, but:
1.) Is he doing anything unethical/immoral to get his pot (stealing, prostitution, running/middleman-ing)?
2.) Has there been a marked change in his behavior/attitude? Have his grades slipped? If he had a job, does he still go to it? Does he hang out with a different group of friends?
3.) Are there any other signs that smoking pot has negatively impacted his life?
Honestly, I'm a *very* recreational pot smoker (once every few months, really) and I've never had a problem with stopping. I have gotten sicker and in more trouble because of drinking than I have smoking pot. I would say that if smoking pot is makiing him an insufferable ass who does nothing but watch Mary Tyler Moore reruns all day, then by all means, get some outside help. If he's doing it on the weekends with a few friends and continuing to maintain a normal life otherwise, then I don't think it's anything to worry about.
I do think that it's worrisome that he's been hiding things and such, but I think you have to weigh all the peripherals and decide how much of a problem this is. My mom, too, smoked pot when she was my age (and younger!), and although I didn't start smoking until I was out of the house, I don't know if I'd want her to know that I smoke up from time to time.
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Well, the God I believe in isn't short on cash, mister. --Bono
But a day will come
In this dawning age
When an honest man
Sees an honest wage.
--The Edge