skipping here from the op, so I'm not sure what's been posted...
This is a VERY common situation in marriages. Dh and I went around and around this for well over a year, in pretty much the situation you describe (though for us, it was a fourth child).
What finally helped was going to therapy. Our marriage counselor was able to talk about why were were feeling what we did, how it often plays out with other couples, and all kinds of conversations/thoughts that dh and I wouldn't have been able to access without her viewpoint and expertise.
Shortly after therapy, dh said he wanted to go ahead with it. It hasn't always been easy...sometimes he feels a little like (in tough moments) he's dragging his feet a little, but over time he's more and more enthusiastic. I'm 20 weeks pregnant, and expecting a baby in September.
The rough outline of what our therapist said (if it helps), is that men rarely understand that women feel there is another child out there for them, and that it's very possible--if they don't have that child--they will regret it the rest of their lives. Men don't get that. They just don't.
She also said that, even if a man doesn't want to have a child, he can still want to give his wife something important to her because he loves her. And really, having a child out of love (if not through self-interest) is NOT a bad thing. She said in marriages that are healthy, and that's a big qualification, that having a child that the woman wants and the man "doesn't" often ends up perfectly fine. The man/partner falls in love with the child and later on can't imagine life without them. However, if the marriage is NOT in good shape, this decision and course of action is often a big, big mistake. It makes things worse, not better.
Best of luck to you. I know it's really devastating to feel so differently about something so important. I'd never had that kind of mental and emotional division from my husband before, and it was painful.
ETA: another good thing about therapy was it really helped me understand dh's position on this. He, too, didn't want to go back to the toddler stage. Our kids were 2, almost 4, and almost 8, and he was really loving all the independence he had....time to work out, time to go golfing, time to hang out with his friends... He is a really, really involved dad, and I think the neborn periods are pretty exhausting for him.
Part of moving ahead was talking with him and with the therapist about his feelings, and not dismissing them. Rather, we discussed ways for him to maintain his independence, his "me" time, while parenting another baby. When push comes to shove, he didn't want this baby to mean another two-year backslide in all the things that make him so happy. I get that.
In a way, he was fighting for what makes him happy, and I was fighting for what would make me happy. A therapist helped us see that, if we were careful, loving, and respectful, we could do both.
DH was in complete shock at first, and took a few weeks to come to terms with it, but now he adores all 3 of his sweeties, and we ended up having a pretty easy going baby too...this one sleeps SO much better than the first two did!
