Once someone starts getting into heavy alcohol and drugs they can change very dramatically in every way. chances are he is on a very bad downward spiral and i would stay far away from him unless you want to get pulled down with him.
there are so many great guys out there that would be ready for a relationship that there isn't a really good reason to stay with him thinking he'll change. he may never change. and you can't remember him as his best, because he is far from that right now and isn't it a place to be that right now.
should you get back together with him? that is up to you, but what i would recommend is to let him be, let him do his own thing. if he wants to do AA and get clean and sober and turn his life around, great. if not, it is his life journey.
I'd just move on with your life. find a new bf if you meet someone. if you find yourself in a place where you are single, and he has been clean and sober for over a year, doing AA or whatever it took to turn his life around, maybe then consider it, but for now, the best thing you could do for yourself is to just move on and let him life his life, while you go and move yours.







