Hllo JoeyUpp, I'm sorry for what's happened as I too also suffered from PND.
If he has cheated on you before then it's not really acceptable for him to contact his ex and wonder what he'd do if his ex had a baby if they were still together, never the less there are other avenues he could have contacted rather this ex, because she, in turn, may be thinking exactly what I've just said.
PNDA is available for men with PND 1300 726 306 where he can discuss how he feels in a better way than talking to his ex and provide him with guidance as this illness can readily happen with the arrival of a new baby.
This was not available when I had it unfortunately, and as much as you want to help him, which is good, you still need to look after your baby as well as yourself.
You can discuss this with the nurses who would have seen this happen many times before and offer you some suggestions, but try and get your partner to have a talk with his doctor, who can then direct him to different other avenues for support.
Please let us know how you get on.