Welcome to SR. I hope you'll take a look at our Friends and Family forums where you'll find the stories of people who have been through or are going through the same thing you are.
Anna is right, you cannot make him quit. I'm sure this all comes as a huge shock to you, but he's been doing this for a long time. Now that you know, of course he's going to say he'll stop so he doesn't lose you. It's rarely ever that simple though. Just as he is an adult and can choose how he wishes to live his life, so are you, and you have the right to decide what you will and will not live with. Making rules rarely works. Giving ultimatums rarely works. Setting boundaries, which are like rules you set for yourself, can be very helpful. Something such as, I will not be in a relationship with a drug user, or I will not allow him or anyone else in my home if I suspect they are high.
The only person you have control over in this situation is you, so that's where you need to start. If you tell him to leave and he doesn't leave, then what? If he lives there, he has as much right to be there as you. These are things you need to consider when setting boundaries.