I, too, had a lot of anxiety about having my 2nd child, and I think it's totally normal. Having a second child is as much (if not more) of a life change as having your first child. It sounds like your concerns are stemming from the love you feel for your first child. You don't want to rock his world, and you don't want to short-change him on love or attention.
As young parents, we tend to think of our children as little ones, so I'll offer another perspective to think about. My ex-husband was an only child, and he LOVED being an only child. In fact, he loved it so much that he almost convinced me not to have a second child. However, both of his parents died by the time he was 25 years old, and his idea of family died with them. He has nowhere to call "home" and no one to share childhood memories with. He feels very stranded. Sure he has aunts, uncles, cousins, and friends, but it's not the same. My point is that you need to think about what's best for your son in the long run -- beyond having to share his toys with his baby sister/brother.
I can't tell you what's right for you. I can only tell you that I have NEVER regretted my decision to have another child. I can't imagine our family without her, and I get sick at the thought that I almost chose not to have another child. I've learned that I have enough love and attention to share with both of them, and the love and attention they get from each other is more than I could ever offer. In most cases, the benefits of having a sibling (in the long run) far outweigh the negatives.
Don't think of this as taking something away from your son. Instead, think of it as giving him the greatest gift ever. Your son is young enough that he will probably not remember what life was like before his sibling came along anyway. Will they ever fight? Sure. Will your son ever say he wishes his little sister/brother was never born? Probably. But, they will also share a bond that can never be matched by a friendship or other family relationship. The first time you see them holding hands or playing together or saying "I love you" un-provoked or kissing the other's boo boo or sharing an inside joke or snuggling together in bed, you will know without a doubt that you made the right decision.
I wish you the best of luck.