OP - it's really painful when people love each other but want different things.
As PP have said, there is no compromise between no children and some children. If you both wanted some children, then you might compromise on the number, but having children changes your lives so fundamentally that having one child cannot be called a compromise in circumstances where he doesn't want any. Given the huge upheaval children bring, if you have one, you might as well have two. And he knows that because he's had children and done the early years parenting. Also, if his children are significantly older, he will have experienced the "getting your life back" which comes with having older children and I can well understand why he wouldn't want to embark on the journey again. A great many parents of older children (probably most) would not choose to do so, no matter how much they love their existing children or their new partner.
It is true that, if you leave him, there's a risk that you won't find another partner to have children with. But you know that he won't have children with you so if you stay with him you are making a decision to give up motherhood.
The other risk you face if you stay with him is this. Given that men generally remain capable of fathering children well into their 40s and 50s and even beyond, it is not beyond the realms of possibility that you will forego the chance of motherhood, only to have him leave you for a younger woman with whom he proceeds to have children. It may well be a small risk in this case but that is a risk any woman takes if she gives up her chance of having children to remain with a particular partner.
So you need to decide whether it's a dealbreaker for you. What's more important to you - having children or being with him? For me, it would have been a dealbreaker for me if my now husband had not wanted children and I would have contemplated going it alone, if I had been unable to find a partner who wanted children. My desire for children was very strong. But everyone is different and only you know how much this matters to you.