Welcome to SR, although I am sorry for what has brought you here.
Do not let your AH manipulate you into feeling guilty, that is what he is trying to do. How many times did he choose drugs over his family? By pointing the finger at you he is trying to shift the blame away from himself. I agree with others that say you did the right thing by moving away from him.
Does your son still have contact with his dad? When addicts are backed into a corner or upset, they can become very mean and hurtful. He might be telling your son that this is your fault and trying to manipulate him as well.
I'm glad that he's in therapy and hopefully that will help. Try to remind him that his dad is SICK and that is was not a healthy environment for either one of you. People that are sick need to get treatment and he was not willing to do so. Try not to badmouth him, though, because it's still his dad.
Keep coming back and hang in there. You are not alone.
Hugs