I would be really careful using something like your pregnancy as leverage against his drinking. I'm not saying this as a comment on whether you should abort or not, that is entirely your choice. That choice should be made for yourself though, not in response to him. If you feel you cannot raise another child, particularly in a situation where he is in active addiction or you are left alone, then the choice is entirely your own. But putting it on him in that way "if you drink then I will abort" will only hurt you both.
When he's in active addiction, he's not thinking about your welfare, your children's welfare, or even his welfare. His primary concern is feeding and protecting that addiction. If you say you will abort if he drinks, what if he continues to drink, but you don't truly want to abort? Either you keep it and show your boundaries are threats you may not follow through with, or you abort and that seed of resentment about that loss is firmly planted.
I have no answer for you as to what the best choice would be. I've never wanted kids, and besides my stance on abortion have no frame of reference for what you are going through. What you said just stuck out to me though. I don't mean it as condoning or condemning whatever choice you make. I only wanted to comment the choice should be made entirely for yourself, free from his influence. I know it's impossible to completely separate him from that choice, since you need and want his support in this. I just meant that he is going to continue in active addiction until he is ready and committed to recovery. He has already shown with your first child that being a parent will not stop his drinking, which means another child will likely also not stop him. If you base your choice on the hope he will stop, it is a recipe for disappointment. Take him as he is now, addiction and all into account, then make the choice for yourself.