A few points then I must 'close' for the night.
A woman wants a child. She has to get sperm from somewhere. OP has thought of where she might get some IF she decides to have a 2nd child. As far as I can see she hasn't said HOW she'll get the sperm - she needs to ask the bloke who happens to be her ex. I'd say there are potential emotional problems in shagging an ex in such circs, but what do other women do for sperm without shagging a man? Yes, turkey basters/ syringes are a bit clinical and impersonal, but an option all the same. But that's just part of the dilemma.
Why do men have children? Not just because a woman asks them. Many/most (I haven't asked them all so not sure of stats
) do so becasue they want to be parents. Lots of men who have one child want a 2nd. If they can afford it, what's the problem? Yes, in the circumstances it's an unusual set up. BUT who says what's 'normal'? If both potential parents want another child and that child will be loved and cared for, and as much as his/her older sibling, whatever the set up, is that not the most important bit?
Obv said bloke might not want another child, so could say NO. Or not with his ex. Or not in this set up. That's his choice. End of story.
Has OP said anywhere she's be expecting more money from him? Does she need more money from him? OP has FT paid work, presumably with mat leave entitlement. She doesn't necessarily need further financial input frm him.
OP, he might need a while to think about it, if it's not an outright no. A lot for you to decide, both together.
FWIW I have 4 children, all with same father - snoring on couch as I type. Lots of people have assumed they haven't all got the same dad. It doesn't bother them or me. it's nobody else's business in the end as long as the children are loved and cared for.