I don't get it either. Are you going to be raising the baby together or not? If so, it starts now. He loves and wants you, then he loves and wants the baby. How much discussion have y'all had about this? If you argue, does he pull out the "it's not even MY BABY!" thing? Or is he just acting as the father already? If so, I think you need to let yourself go with it. Let him be that.
My oldest daughter is not my husband's (which makes it somewhat different I know, we don't have the complication of it happening while we were separated). But he loves her just like he loves the kids we have together. And he lays out money for her just the same. When we go out to dinner, he doesn't tell me that I need to pay her part out of the child support her dad gives. We have savings accounts for each of the kids, and DH puts the same amount in for her that he does for our sons. She has a dad for sure, but DH also supports her, emotionally and financially.
Does your ex intend to be involved?