This is where my snark would come out. That's probably not useful but then... maybe he needs a wake up call.
1. YOU are not the jerk here. Seriously? Men who pull this stuff should be publicly flogged. (When I was PG the guy I was with used to point out what the 15 year old speed freak mother of his first child did. No, really. Underage dope addict was the standard I failed to meet. Maybe I'm biased.)
2. You have to get across to him sooner rather than later that this behavior is not okay. If being polite doesn't work, be openly hostile. "I don't care what she did." "SHE isn't the mother of this one." "Yes, but you're married to ME now...remember?" "Sarah is not here now. I'm Mary." "If she's the perfect mother, why did you have this one with me instead?"
He's being a tool. If you have to tell him that in so many words, I say do it.