OP. You asked how do you co-parent with someone like that. I think you use the skills/tactics/behaviours that you need to employ to leave him.
You take a purely adult position. You disengage as much as possible, so you are only left with the practicalities. Once you are no longer a couple the only focus and topic of conversation is your child. Anything else is ignored. Don't be drawn into anything about your past relationship.
Set up a separate email account for discussing arrangements with him as regards seeing your child. Block him on everything else. This way you don't have to worry every time your phone pings, you can keep it separate.
He may well use seeing his child as another way to try and control you. But he can only try. You can resist. It's bloody hard, but you can do this.
As for knowing your relationship is not right but not being able to fix it. These situations are very complicated. The behaviour is very subtle, it's a gradual creep, very manipulative. They do just enough to make you think they really care, and that in fact they're the victim when you don't dance to their tune.
It's almost a script. It doesn't matter if he knows he's doing it. He is. You've also now told him how he makes you feel. He hasn't stopped. He doesn't deserve you, or a family life with his child.
You're much tougher than you think. He's weak and pathetic.