Do anything it takes to get yourself out of that situation. Anything. And do so before you bring law-enforcement into your domicile. That will probably be costly and difficult -- face the facts, bite the bullet, and GET GONE. Right now, it should be your absolute first priority.
Consider that the consequences of being caught up in any criminal proceedings -- from your actions or from some random event that could befall a drug abuser at any time -- will ruin a significant part of your life, for a long time to come. Remember as well that, "That's not mine, it's my roommate's! I don't know how it got in my drawer, HONEST!" is the single most common expression a narcotics officer hears during the course of his duties.
Get OUT. Once you've gotten clear, THEN take such proceedings as you feel are warranted.
And I'd give him NO warnings or any indication that something is up. He isn't going to quit smoking, he isn't going to quit risking your freedom and liberties through his actions. He certainly has the ability to harm you, contaminate you, and drag you down with him, if he suspects you'll bring in the law. He's already got you pretty legitimately over a barrel right now (you can't call the cops for risk of being implicated yourself). That's only going to get worse unless you can get yourself physically extricated from the situation.