Yes, it really is letting the child down as well. I hear this from men, "I wouldn't marry because when it inevitably ends I don't want the ex to get her hands on my money - but I'm a decent guy, I'd look after the kids!".
Firstly, yeah right, as if they are likely to step up and do the daily grind, not just the fun stuff.
Secondly, where will these kids live - surely either mainly with their mother, or half the time. And where is the mother going to live, in the poor financial position she's been left in?
If these men really prioritised their kids (actual or hypothetical), they would also look after and respect the mothers of their children. Because that has a huge impact on the children in many ways.
OP - To give this prick the absolute benefit of the doubt and try and salvage this, it might be that he's insecure about the relationship (e.g. feels left out now that baby's here), and instead of communicating this constructively like an adult, he's just decided that the relationship might end and therefore he needs to protect his interests. You shouldn't have to pander to this level of immaturity, but given the alternatives, it is worth having that conversation with him. Tell him he's clearly less happy in the relationship than he was when he proposed, and you would like to approach this together as a problem to be solved. Don't let him fob you off. If he won't engage honestly and constructively, then you have your answer.