I'm in a similar position, but I'm the one wanting a second while husband doesn't.
The person who doesn't want a child wins. End of story. Whether that's child one or child number ten. You need two yeses.
I'm sorry but him saying you might split up if you don't have another strikes me as quite manipulative. I'd be lying if I didn't say there are times when I consider how badly I want another and whether it's worth ending the marriage and starting a fresh or not, though I always come back to no, it's not worth it and I'd rather have a family of three than split up our family for the possibility of a second with someone else. But I would never say that to my husband, I would quietly make that decision and inform him if it ever came to it. Otherwise it's a threat, it's basically saying 'give me another child or I'm leaving' and it's not fair to ANYONE to have a child you don't want!
MN can be weird about this topic, I've seen people say stuff like 'I'll be damned if I let a man tell me whether I can have a child or not' or 'no way would I allow my husband to dictate how many kids I have' but the fact remains that either of you are allowed to say no, you don't want another child, and that's how it must remain unless one of you changes your mind.
If he wants it badly enough he's willing to leave then let him, because the resentment will run so deep it will destroy the relationship anyway. But even that outcome is preferable to having a child you don't want. That's not fair on you or the child or your existing child.
If he says 'it doesn't change what I want' then simply say 'okay' and walk away or change the subject. It doesn't matter whether his mind changes or not frankly, because he doesn't get to insist you have another child. He can have one with someone else as is his right but he can't make one with you unless you want it. My husband wanting to stop at one hasn't changed what I want but neither does wanting a second mean I have the right to have one WITH HIM. We all get to decide for ourselves what we want and if that makes the relationship incompatible then so be it.