To be honest you shouldn’t try to force a man to have another kid just like men shouldn’t force a woman to. You really only have a few options 1. Be happy with the lil family you guys have now. And maybe a couple years down the line he will change his mind or he wont. 2. Really decide if having another kid really matters to you or not and leave the relationship in search of a new baby daddy lol 3. Adopt a boy?
Probably try to know what are his apprehensions about 2 daughters , once you know why he is behaving that way or if there is a fear in his mind , you might be able to help him overcome it .