Please take care that you are not spending precious time -- time that could be spent focusing on the child you DO have -- in pining for a child you don't have.
I have seen women do this, and before they knew it, their only child was in elementary school and off doing a hundred activities and...no longer very interested in being with mom, learning from mom, spending time with mom.
Embrace what you do have. There is no reason at all to feel guilt over his not having a sibling -- feeling guilty is a waste of your energy. We should never, ever provide a sibling as a playmate or potential friend for the children we already have -- if we don't want a second child for that child's own sake, it's wrong. A sibling might not get along with your son at all; they might have utterly different personalities and interests; they would surely fight at times, and might fight much of the time. The warm and fuzzy image of perfect little siblings adoring each other is....an image, not reality, not for everyone.. There are siblings who are close and share things -- but you cannot guarantee it.
Don't waste the time you have with him. Don't pine. Go out today and do something that only you can do with only him -- read together, play a game together, go outside and throw a ball together. Don't think, "If only he had a brother to read with, a sister to throw a ball with." He has YOU.
Ask yourself, objectively, how much of the pining is for a family with a husband and dad in the picture and how much of it is really for another baby?
I have an only and have zero regrets about not having any others. I know her very well. I have time to do things with her that I would not have if I had younger kids to deal with. I can do things for her and participate in things with her in ways that would be impossible if I had more than one, just because of logistics and finance. No regrets; in fact I'm glad I have just one. Do what you can to take advantage of the fact you have one child, which buys you time that you would not have if you had more.