I won't comment on the child he doesn't see, as I've no experience myself and it's not really what you've asked.
With regards to another baby, I'm in a similar situation with my boyfriend. We have a daughter who's 6 months old, she wasn't planned and he never wanted children, which he was always very open about (I know that's different to your situation)
It's taken him time to come to terms with us having dd and he doesn't want more.
I'm a only child so definitely don't want dd to be a only child though.
Any conversation we've had as kind of come to a stalemate, of one of us saying we'll do what he other wants to avoid breaking up. To be fair it's always him agreeing he'd have another, if it's what I want. I don't want another in that way though.
As difficult as it is, I've issued a ultimatum in a way. He's got until I finish uni, 3 years from September, to know what he wants.
I'm obviously hoping, between now and then, he loves being a dad so much he'll want to do it again.
I know in my heart, I won't be happy not having another child, so it will be a deal breaker for me.
I'm also only 19, so have got plenty of time to meet someone else, if it comes to that!
I know it's not easy, but I think you need to think about what you'd miss the most. The chance of another child, with someone who wants one or your partner.
Also, you need to think of practicalities, if you spilt you'd have shared care of your first dc and he'd potentially have a step parent. Is that worth another baby? You might never meet someone else, or not someone who ticks all the boxes, what then? And of course age can be a factor.