I, too, would like another and my husband is also adamant about not having two children. I know many friends that "talked" their significant other into one or two more (one even using the puppy argument, "I promise to take care of it and you won't have to do a thing!"), but I truly think that while you can talk someone into a paint color or even a car, both parties must be on board for the full responsibility of a new life from the beginning.
So...my son will be a singleton.
Turning 40 this year also puts a bit of time limit on my husband changing his mind. While I think 40 is definitely not too old to start or grow a family, one of his fears is the higher risk of complications that could arise with advanced maternal age. So that's not going to diminish the longer we wait! I miscarried 18 months ago and after that, my husband is TERRIFIED that I will get pregnant again. He'll barely shake my hand. ;)
While I sometimes grieve not having what I didn't even realize I wanted, I also have to comfort myself that it's not the end of the world or our family for any of us. My son has loving parents and friends. I still get to see this amazing kiddo become his own person. We will continue to grow as individuals, as a family and as a community. We will have experiences that families of all sizes have and special ones that only the three of us will share.
It still hurts sometimes, and I find myself gravitating more towards the friends of his that have no siblings so we'll feel "normal" with them.
But no matter what, I'm lucky to be a mom.