A thing, it’s fine for men to spout they want a child but it’s not their body that gets to carry, birth and feed it, they don’t take a long maternity break in their jobs, they are not usually the default parent, how many men walk out of families when they get bored?
You are late 30s OP, there might be, also might not, but statistically, an increased chance of complications with pregnancy, birth and baby.
Would he stick around if his legacy child wasn’t “perfect” in his eyes? He might say he would but would he really?
It’s difficult as it sounds like he has and does step up with your children, he said at the beginning he wanted his own child, he is aware that your fertility might decline and you said you were open to the idea.
On the flip side you know what single parenthood entails, don’t want another child in precarious circumstances and want the security of marriage as a commitment.
Personally I wouldn’t have another child in your circumstances. But I would tell him that directly. If you are open to the idea with conditions, you need to talk it through.
If he wants his own child he is likely to leave, or resent staying. If you both have a child and split up you will resent him.
It sounds like a lot of your relationship is good, so you need to find a way forward without resentment on either side. And that might mean splitting, but it could be done amicably.
It’s just different for a woman having a child. You are wise to put in conditions and security, particularly financial before agreeing.