I think wether or not to have more children is something we all tend to overthink, and all the overthinking can drive you mad! In my opinion, I think it's best to just see what happens -- b/c no matter what, you will adapt your life to whatever happens. If you have another child, then ya'll will adapt, and if you don't , you'll adapt to that also. You'll "work with what you've got" and no matter what you choose, that will be the "right" life for your family. As long as you and your husband are loving parents and available to your son when he needs you, he will not miss out on anything wether he has a sibling or not.
Personally, I have 4 children. The first two were not planned, the third was planned, and the fourth was almost planned (we finally agreed to have a fourth - but by that time I was already pregnant only we didn't know it, lol). Mine are 11, 9, 5 1/2, and 3. They are all very close, even the 11 yr old and 3 yr old.But that is how we raise them and how we run our home, we are a family unit that only works right when we work together. I love my children dearly, and they love each other dearly, they do have their occasionall squabbles, but that's life. For us, we wouldn't have it any other way, and we do still plan on adopting in a few years. For us a big family works and we love it that way.
I have a brother who is 1 year and 2 months older than me, we did NOT get along as kids, and did NOT get along as teenagers. Now as adults we finally get along. It was never about our age, it was just b/c we had a very bumpy lifestyle. My husband has a sister who is 13 years older than him, they get along great.
I wish you the best of luck in whatever you decide! God Bless!