Leave him now. He is a looser who will only bring you pain and sorrow and regret. Leave him. Leave him. Leave him.
If you want a boyfriend who doesn't do drugs, you need to find someone else. The only way drug users stop is to do it for themselves, not for somebody else. May I ask how you got to be with him if you're so anti-drug? Cocaine use would be an absolute deal breaker for me.
What you are describing here is classic addict manipulation of you. You need to get away from this lying loser and never have anything to do with him again. Addicts are the worlds best liars and manipulators of those who care about them. GET AWAY and NEVER GO BACK. NOW!
You need to leave. If he really wants to get over his depression, the are many, many LEGAL, NON-ADDICTIVE, prescription drugs he can get to help him get over his depression, and none of them costs as much as any of the crap he's putting in his system right now. I've lived with people with chemical dependency problems all my life. The person I live with right now has almost 30 years clean and sober. Because he wants it, he busts his @ss working for it. When we moved in together (over 20 years ago) we made an agreement that the two most important things in our life together would always be his alcoholism and my mental health. You really need to get out of this: I know 5 years is a long time, but if things stay like this, you'll be looking at 10, 15, 25 years of knowing this isn't where you want to be. Please, be the strong person you know you are and move on ith your life, away from this loser.
You are so right.