I would try to ignore him or respond underwhelmingly? I had a boyfriend who kept calling me long after he married and had a child and I married and he lived/lives in California and I'm in Maryland. I didn't want to hurt his feelings by not responding at all but I did say things to him I don't think he wanted to hear; maybe you can do something like congratulate him on the pregnancy but tell him you wish next time he had news he would tell his family before he told you (or, I wouldn't really say much at all, maybe a one-word, "Congrats" which would not be the interest I think he's fishing for?).

Just remind yourself why you left and how you didn't feel the relationship was working out between you; my boyfriend turned out to be more self-involved than I liked and did some things I couldn't respect/admire. So, it wasn't hard to be a bit cold when he caught me at home, calling me. I would specifically ask where his wife is but cut him off if he complained about her behavior, sometimes he would call when he was home alone babysitting their child and I'd be all bright and breezy about "you must have to go now, you are babysitting remember!" to get off the phone with him.

It's not about you, it's about him. He needs something he's not getting from his wife and you're the next person he is trying to get validation or enough/extra. Be more emotionally unavailable but not in a way he can complain about and he'll wander off and look elsewhere for whatever it is he's looking for?