I've never smoked it, so I can't comment on that, or anything... but I will say that you probably have a better chance to watch out for him if you don't strongly discourage it or anything right now. I mean... your odds of suddenly convincing him not to try it, if he wants to, seem pretty slim. And after he tries it, if it ruins his life (assuming he doesn't realize it--and most people don't, for a while), he'll always remember you as that person who didn't respect him and tried to act like his mom, when you aren't.
Whereas if you don't try to force it now, there's a good chance he'll keep his respect for you, and you can see where it goes. If it's not a problem, then it's not a problem. If it is a problem, you can say "look at all the ways you've changed over the past few years.... do you like your life heading in this direction?" -- and he's more likely to listen to that, since it's talking about things that have been happening, rather than things that "may" happen.
...that's at least how I think I'd think about it, anyway. I may very well just be making excuses, though. It's probably a complicated game-time decision that has to be made, based on how likely you think you are to convince him never to try it (taking into consideration how sure you are that it's a good thing you convince him never to, etc), how likely it is to ruin his life, and whether he'll listen more to speculative argument and risk/benefit calculations, or empirical evidence of his change (and memory of what he used to think like, and want out of life, and things like that)