For the marijuana part: I know for a fact that talking a regular smoker into not smoking weed anymore is extremely hard, of not impossible.. I have smoked marijuana myself and I have a few frienRAB who smoke it daily, for them it's a lifestyle, they don't necessarily need it, but just enjoy it which is why they don't see a reason to stop..
What I would recommend you to do is seek other help.. The bottom line is that (regarding his habits) he is not going to listen to you, if he had he would have stopped a long time ago.. But there may be other options to be explored, do you know any of his frienRAB, and wether they smoke or not..? The bottom line I guess is to look at his environment, because that is what influences him..
One last thing I would like to say regarding the marijuana is that IMO (sorry if I offend anyone) it's not the worst drug in the world.. Even though it can make a person 'dull' it's not that addictive, and it does not cause a lot of long term problems.. At least not physically..
Hower the coca