I am 21 and have been sober from meth, pcp, heroin, and marijuana for a little over a month now. Marijuana has been by far the most difficult psychologically to quit. Especially with it being legalized and being a more widely used drug. All of my friends smoked weed, but one thing I did realize is that if they are your true friends they will support you quitting. I know it isn't the case for everyone, but I used to rationalize being a pothead and saying "it's not like I'm a meth head, there's nothing wrong with it" and I never pictured in my wildest dreams that's what I'd end up becoming. Another thing to consider is does he really want to stop? "it's full of old people" sounds like another excuse to justify his actions.
I hope your son does get sober, but it's really his choice. I think at first the best thing would be stay away from those friends to minimize any temptations, show them he's serious and then the true friends will emerge to support it. However, it is all his decision. Good luck, if he's ever interested in talking to someone closer to his age I'd be happy to go through my experiences.