Sit him down. Smile. Kiss him on the forehead. Inhale. Exhale. And say this:
"Honey. Snuggle Biscuit. Sex Jedi. Here's a cold beer. And I nuked you some mini-egg rolls. Are you comfortable? First of all, I love you. I trust you. But as someone who loves you, I feel I have a responsibility to warn you if you're doing something that I think is unhealthy. That is potentially self-destructive. For instance, If I were to start sniffing airplane glue, I imagine you'd sit me down and tell me to stop. Right? Right.
So what I'm about to say might upset you. I get why it might. So let me assure you again: I trust you. I don't care who you text, e-mail, or friend on Facebook. But if you don't stop talking to that ex-girlfriend who faked that pregnancy and who spreads vicious rumors about you, I'm going to beat you with a pillowcase full of kielbasa. I say that with love.
This woman is a tornado of crazy. A succubus. She's a leech woman. A troll squatting on your chest and stealing your breath. You don't owe her anything. I'm sure it was exciting to be with her, but, you know, that moment right before you drive a car into a brick wall is pretty exciting too. Lots of adrenaline is firing, I'm sure. But then there's the sudden stop, the shattering glass, and tthe steering wheel taking your head off, Let me add here: she hasn't changed. Because if she has, then she would have said her piece to you, and moved on, leaving you to be happy with your awesome girlfriend.
You need to stop talking to her. Not because I'm jealous. But because not only is it unhealthy or you, but it's terrible for us. I'd rather not watch you resume contact with someone who does not have your best interests at heart. Because I do.
I'm not going to mention this again. Everything is cool between us. I'd tell you that I love you again, but you know that.
Want another beer?"
If that doesn't get through to him, and he continues, you might have to think about pulling the ejector seat. It's not uncommon for people who have gone through dramatic relationships to miss the chaos. Eventually, the ball is in his court and you're going to have to make difficult decisions about your own life and well-being.
For the record, I am believing you when you tell me that this ex of his was as bad as you say, and that you have no problem with him communicating with other women from his past.