T.B.
For some of us, we're never really "done" in our hearts. I mean, logically, I know my family is complete. But in my heart, I can picture a new baby in an instant.
For what it's worth, your son is only 18 months old. He is still a baby! Families come in all shapes and sizes with siblings all sorts of ages apart. There is no guarantee that having a child close in age is going to ensure that they are friends and there is no guarantee that having them further apart means they won't be. Perhaps when you guys next speak of family size, you can open your heart and minds to the idea of a second child when your youngest is older? If you feel that he is truly on the fence and don't want to hold him back from his dreams of a larger family, then I suggest that you personally try to picture what another child might feel like in a few years. I say, never get pregnant without being sure, but closing a door on a baby may be premature right now.
ALSO! One child is fine. There are benefits to every family size and a singleton has advantages that a child with siblings does not. My own siblings are my best friends and I cannot imagine my life as an adult without them, but that is not always the case.
Best of luck.