My heart breaks for you, it really does.
I agree with Carl. Nar-Anon for you. Rehab for him if he decides to go. You cannot make him and I would guess ultimatums don't hold a lot of weight with him right now.
Many moons ago, I was in what sounded like a similar relationship. I did drink a lot with this guy, but I found out later on he was into hard drugs. Instead of finding it in his wallet, I found a powder residue on my sink after he left. He said it was only once (after telling me I was crazy- it must have been bathroom cleanser). His behavior became increasingly erratic, emotionally abusive, and exhausting. I spent all of my time worrying, wondering, obsessing over getting him better. If he did I'll never know. There are many situations where I could have lost my own life because of the dangerous situations he put me in (that I allowed him to put me in). I could go on and on with outrageous stories but the result is still the same.
If I can be blunt, your BFs using means everything to him. And trust me, he is using and lying about it. His words right now mean nothing. He will tell you what you want to hear.....he knows the script by now. My honest opinion, which is harsh I know.....run- in the opposite direction, fast and furious. I'm guessing you are a fairly young lady with a great life ahead and career goals. Please don't throw it away over someone else's abuse of substance and abuse of your kind-heartedness.
There is also a Friends and family area on this site.
I wish you well.