You cannot force him to go, nor would it be a good idea if he is resistant to it in the first place, because it won't help. He has to decide he has a problem and want to do something about it. You can speak to him about your concerns and how drug use is affecting your relationship, but it has to be his decision. The only thing you really have control over is whether or not you will put up with it, you have the option of leaving him.