m0vinf0rward Your post is incredibly unfair.
Op, I have no advice. He was really unkind to dangle this as an option when you had resigned yourself to accept you were finished.
Actually, I would suggest you decide when is the latest you will be happy to have another child. Give him this information very clearly, and then leave it at that.
Eg, “I respect your right to say no to a child right now, equally I hope you respect my right to have a ‘cut off’ point of when I am no longer willing to go through another pregnancy. So the options are that we have or third child before or we go back to the original plan of finishing our family at two children. But I need to be clear that I will not be having another child any older than .”
That is not a discussion. Or a negotiation. That is providing him with clear information. It is setting your boundaries and getting away from this really questionable dynamic where he feels he gets the final say in if and when you will grow another child for the family.
You have stated your boundaries and beyond that it is up to him to decide between the two options you have offered and you must be willing to accept that either is a possibility right now.
I don’t need to say, but the one thing you can’t guarantee is to get your last baby in your preferred timescale. Both parties always have the right to say, ‘no more kids’.