Clay, do everything you can to find something to occupy your DS during the holidays. A paid job if possible. Voluntary work if you can set it up. Travel. An expedition. What exactly this should be depends on your DS: the crucial thing is that it should be something that motivates him and that he needs to keep his head straight for.
IMO there are two entirely different kinds of dope smokers: ones who smoke to unwind from whatever it is they normally do, and ones who smoke to switch off from life because it's boring or empty or awful. The first lot seem to be ok. The second lot seem to have problems, sometimes catastrophic ones.
The difference between the two groups seems to be whether or not they have something else in their lives that motivates them.
You don't get to choose or control which group your own DC will fall into, but you do get some control, usually, over helping them find something that they want to do.
I speak from personal experience. My DS1 was in the second group from 13-17. It became increasingly grim. At first he was just one of the spliff-smoking kids in school uniform that Maryz describes. Then he was taking other stuff too. Then stealing from me and being violent. Then being arrested on suspicion of burglary. Then his dealers were arrested for murder... Desperate times.
I did everything I could think of. He had no allowance for almost 2 years. His room was stripped, partly because I'd confiscated stuff, partly because he'd sold it. I even had him arrested. Ultimately, everything reinforced his feeling that his life was shit and skunk was the best thing in it. :(
In the end, there were a number of 'turning points': his dealers went to prison; he stopped taking m-cat; he got onto a college course he wanted; I moved us; he got a part-time job... Over the course of a year or so, life for him changed from something that felt awful and out of his control, to one that he began to like.
He still smokes dope, but now only at evenings and weekends, on his way home from work rather than his way to school. There's a bit of me that wishes he didn't smoke at all, of course; but on the other hand almost all his peers do - and all my friends' teenagers, and many of my friends, so it's a pointless wish... Better to be happy and relieved and proud that he's come so far. :)
Don't let him grow it, Clay. The arguments in favour - that home-grown is better than skunk and he'd avoid dealers - are outweighed by the arguments against - that it's illegal and risky and could cause trouble for you as well as him. Ultimately you can't stop him making bad decisions for himself, but this is one that would involve you too, so that's a line you get to draw.