I'm sorry you're hurting. I don't really have any idea why we sometimes claim we "love" someone who's a drunk, a drug addict, who hits us, belittles us, screws around on us, uses us, manipulates us. I know it happens -- 'cause damned if it hasn't happened to me too! -- but I'm not a psychologist and I don't know what it is.
But it's not love. That much I know.
You sometimes crave him, just as he craves his drugs and booze. I know what that feels like too. It's hard.
But you're going to have a son now. And if you are carrying around any doubt at all that you've been given an opportunity to save your son's life, put it away: this is it. you're standing at the threshold of either allowing this man to permanently screw up YOUR child by being around him, or overcoming your addiction to him somehow (counseling, reading, finding someone healthy, whatever it takes).
As someone who was raised by two addicts addicted to each other, who wouldn't leave their relationship and so raised me in addiction, neglect, violence, and anger, I can tell you this much: It took me more than 20 years of the most acute suffering you can imagine to even BEGIN to recover from what my parents did to me in the name of "love."
If you would like to see your son 20 years from now IF you permit this man back in your life, step over to the Friends and Family of Substance Abusers forum and talk with some of my friends there, the parents whose kids are destroying their lives with addiction.
I want to be the kind of person who can give you hugs and compassion because (for whatever reasons) you miss something that once lived in this man, and missing someone is hard as hell. But it's hard for me, because I know what that child's life will be like if you give in and let him back in. I can sit here and feel the tears of sadness and anger welling up in my eyes for what you're going to put him through.
Good luck with this. I'll pray for that little boy. He didn't have a choice in this.