First of all, I'm very sorry you were put in that kind of situation, I hope you find a good resolution.
Second of all, as said, the ultimatum might not be the best idea (I'd say use it as a last ditch effort if all else fails). What I would try however is just to sit down, have a long talk. Thing is, you have to explain to him how you feel about him, and how his self-destructive behavior is not hurting only him, but also hurting you. I guess the idea here is to convey your feelings for him as best as you can, at that point I'd guess one of the two things can happen.
1. He realizes you are right, he realizes that you're worth more than his experimentation (let's be honest, mostly negative experiences are far from what most here would consider recreational use), while he might not instantly change his mind, it'll probably prepare the ground and you two can work on this thing together and over time, hopefully his desire will disappear...
2. This one is more painful, but there's an option that your relationship is one-sided, if he doesn't come to the realization what you actually mean to him, it's going to get messy, or that his silly reckless behavior is more important to him than your relationship, then I'd recommend you seriously consider leaving him. My opinion is that if you're not happy in a relationship, it's probably not worth staying in, there's no need to waste several years of your life hoping he'll change his mind, or even worse, watch his descend into more psychotic episodes...
I guess what I'm trying to say is, he has to come to terms what he feels about you and how he feels about the relationship before he's going to make a potentially life changing decision (even if it's for the better).
Anyway, I wish you the best of luck, and feel free to give us a shout how it all turned out.