This is hard. I can definitely understand your disappointment. I just thought that I would share my perspective on it in the hopes that it will help you. I only ever wanted one child. I myself am an only child, and loved my childhood! I wouldn't want to change a single thing about it, and have never missed having a sibling. My opinion didn't change when I met my husband, who argues constantly with his sister, even though they live on different continents.
Fast forward to our first child being born. This darling child did not sleep during her first year of life. It was almost like she was super-human. Who can go for a whole year without sleep? She can. I had horrible post-partum depression (probably because unlike my superhero oldest child, I NEED SLEEP). Meanwhile, my husband was doing the full-court press to have more kids. Needless to say, I wasn't a huge fan of that idea.
He pushed and pushed and pushed, and finally I relented. We had a second child. Now, don't get me wrong, my younger daughter is the light of my life. I wouldn't change her for anything. BUT... it would have been WAY, WAY easier for us to just have one child. Financially, emotionally, in all ways. I love both of my children so much, but I can find no fault with people who stopped at one. My girls fight ALL THE TIME - they are as different as oil and water, and I hate the constant conflict in our household. I wish they could be friends, but it doesn't seem like that's going to happen. So we live in a war zone half the time.
As for my husband and I, originally his master plan was for us to have 3 or 4 kids. After #2 was born, I told him that I was done. He had a hard time accepting that, but finally he went and had a vasectomy and recently he told me he's so glad we stopped when we did.
Enjoy your son. Try not to think of him as your incomplete family. Maybe you got it just right on the first try! =)