Imagine a leash around your neck and BF holding it. When he pulls it so tightly to the point you can't breath he lets loose so you can, then starts they cycle of pulling it until you can't breath all over again.
This ^^^ is your life. When he drinks too much to the point you have had it he stops or slows down. When he does drugs to the point you are ready to leave he stops. When he has ignored you to the point you want to move out he hugs you and pays attention to you.
All you have to do is take the leash off. You control nothing in this situation, he controls it all. You HAVE proof - you don't' need more.
Knowing that his friends do drugs - I am uncertain why you are subjecting yourself to this holiday. I doubt it will be much of a holiday for you I expect it will be a disaster - are you hoping it will be so you can leave him? Instead of taking control of your life and making the decisions that are best for you, a life that you want, you are waiting for the world to help you out and make that decision for you. You are hoping he screws up bad enough to give yourself permission to leave.
Playing the "what if" game - "what if he stops, what if he goes back to how he was…what if, what if, what if……WHAT IF - that game has trapped you as well. Life is not about "what if" its about WHAT IS.
Hoping you take that job and move along. This situation isn't headed anywhere but South and you deserve better.