I am in a similar situation. My bf has a child from a previous relationship that ended up in him trying to suicide(that woman really screwed around with his mind), after a few months of breaking up she told him she was pregnant. Now he has a kid he pays child support to (and I make sure it's on time every month because I really don't want to deal with his ex).
I have some of the same issues you do, with jealousy and thinking about the future. Your bf said he didn't want him and I can understand that in the sense that my bf is trying really hard not to care since he can't be 100% in this kids life. he doesn't know if we are going to move away from where the child is or stay closer for a long time. He would hate to start having more of an influence on him and then just disappearing (He also doesn't want anything to do with his ex, and that is really hard for him since she is the main point of the kid's life). But in your case, I don't know what the reason is he says that. Maybe it's to make you feel better. But you should get to the bottom of it, because it's not ok for him not to care just like that.
Anyway, personally I didn't find a way to deal with it yet, I just realize that every time it comes up it hurts less and less. I know that I'm probably not ready to meet the child but I will have to someday.
You can't say that if he has a child with you it's going to be his second, because it's going to be his first with you. He also probably missed a lot of things that happened with his first child and those will be a brand new experience for him. I wouldn't worry about that so much.
You also have to think that the child will never be 100% part of his life like you are or your children might be. As you said, it's only twice a year (for now of course).
In the end it all boils up to how much you love this man. If you can figure spending all your life with him, then this will just be something you will have to be neutral about.