I dont know if I have advise. I have an 18 year old with a nasty pot habit as well. We have done it all-Residential Treatment Center (RTC), counseling of many types, alternative education and we still have a pot issue. By the way everything, but the violence that Midwestmom posted, is also a sign of marijuana use. It is not always a gateway drug, but can be and if addiction is in your family, you may want to seek more structured help. Day treatment, Rehab. (Esp. If behaviors are an issue).
No amount of restrictions on your part will keep him from it if he wants it and is self-medicating. My daughter swears it is the only thing that gives her releif from her depression and anxiety. She is very mellow on it. We dont approve of it because it is illegal here...but just across the border, where it islegal, it is given for BiPolar (BP), depression etc.
Without knowing more, maybe attending alanon, or families anon. will help you. I have found support there. I dont buy into all of it, but that is the beauty, you take what helps and leave the rest. I agree that 17 is a tough time. At 18, you have almost no power, so its now or never to get rehab. help. Quitting mj is tough.