I understand your sense of helplessness. Your son understands it too and he is using you so he can continue his addiction.
We have to reach rock bottom too before we do what we have to save ourselves. It sounds to me as if you are getting there fast. If your son is anything like mine, he will play on your guilt. You weren't a good mother, good parents support their children, how can you do this to your own son etc. etc. They know us well and know exactly what buttons to push.
How you get to the point where you do the right thing, is where you finally say ENOUGH! Enough of the abuse, enough of witnessing his self-destruction, enough of the manipulation and lies. Once you say that word and you realise you have a RIGHT to say it, it gets a whole lot easier.
Apart from the effect they have on you, it is the others too. For you it is the effect he has on your husband, for me it was the effect my AS had on my other son.
Your AS chose his path. He chooses to stay on it. You on the other hand can choose the path you want to walk. It only has to be with him if you say so. You can choose at any point to stop the devastating effect your AS has on you all.
I walked around feeling like I was a victim of my situation, until I realised I was not. I was only the victim of my own choices. It still wasn't easy initially, but it really gets better.
You will find so much support here, you will never feel alone in your choices again.