I honestly think it's unfair. OP is obviously young and wants to have a family with more than one child that she can raise together as siblings.... it sounds to me as if her partner was aware of this before they had their first.
Op has said there's no issues financially, and that she does majority of the parenting... so why on earth would you want the woman you love to not be able to have the family she dreams of simply because you've done it all before? She hasn't! And he knew that when they had their first. I don't think it's fair at all to just decide now that he's done, and it's the Op's bad luck. I would be absolutely devestated if my husband had told me after having one child that that was enough, no more.