I don't get this at all. I am thrilled to death my wife and I are having a girl. I would be just as thrilled if we were having a boy.
Just because the child is a girl, doesn't mean she won't want to do boy things with Dad. Some girls like to fish, hunt, watch sports, throw the ball around, etc. And some boys don't like that. He should understand that each child is going to have its own interests and personalities. His son may grow up and not want to do the "guy things" like sports and outdoor activities.
And even if his daughter does develop into the stereotypical girl, so what? You are her Dad. You get to be the #1 guy in her life. You get to be her hero. You get to go to Father-Daughter dances. You get to scare away potential boyfriends. You can show her the traits in what to look for in a man by the way you treat your wife/SO/girlfriend, etc.
I know I am preaching to the choir here. I guess I don't have much of advice here. If you knew he wasn't that onboard having a second child, why did you choose to have another one with him? I know that isn't the point because he should step up and be a father to both of his children. It just seems like you had some doubts beforehand and here you are.
I agree with Coltsdad. I don't think this is something that is going to change overnight and I am not sure how to instill the value of wanting to be a parent to ALL of his children. I wish I had more to offer you. Good luck with your situation and I hope he turns around to be a great father to both his son and daughter.