You are one strong women, I love this statement " I know I can do this on my own if I have to. "
Focus on yourself right now and your baby. I'm not sure that I would stop him from being there for the birth. No, we can't be enablers but we do have to remember that addiction is an illness. And right now, there is no cure. If you know that you can have a life on your own, letting him see his son being born would simply be a kind gift and it is his son, too, after all! But again, only you know what's best.
I hope he gets the treatment he so desperately needs and my heart goes out to you, this is really rough!