J.B.
Actually plenty of people regret having a second or more child(ren)! Please don't ever have a child (after all you're bringing an entire human being into the world) just for your existing child. The sibling isn't a pet for your son, and if you don't really desire having another, you absolutely should NOT do it! I'm an only child and I have no issues with it. Except for a blip when I was 5, I've never wanted siblings. I got to choose my family in my wonderful friends who I've had for years now. My mother died last year, and they comforted me more than I've seen most siblings do for one another.
You have absolutely no guarantee that your son and a sibling will get along or share anything in common. I know a lot of siblings who really have nothing to do with one another -including a number of them in my own family on both sides.
My husband is also his mother's only child. He has two half sisters in California, but he only saw them now and then and never lived with them growing up. He doesn't have any regrets about not having full siblings either.
Yes, IF -and only IF you and your sibling or siblings get along and REMAIN that way when your parents age and are dying -then it can be a great help. However I've watched my mother and her 5 siblings and my father and his 4 siblings fight, quit speaking and have a number of arguments and issues over the care of their aging parents, funeral arrangements and everything in between. If I had a brother or sister -it would be nice if they were near by and I knew I could count on them to help as my father ages, but you know what -that's no guarantee! My very best friend in the world has 4 brothers and only one lives anywhere near her parents and her, so their care primarily falls to her. The closest brother doesn't speak to her dad.
If you're financially comfortable -that's HUGE! Plan for your old age and retirement years now. Think of the fact that you have an only child, so you want to make sure you have enough savings to put yourself in a nice assisted living center if you ever need it. My parents did that, and it has eased my mind considerably now that my father is alone and aging. If he gets to the point where he doesn't need to live alone -he has the means to stay somewhere very nice.
I also got to do many things that I wouldn't have been able to do if my parents had another. They took me everywhere -all over this country, the Caribbean and Europe. The schools where I grew up were really lacking, so I got to attend a private school. My mother was also able to stay at home and work a seasonal business my parents owned for about 3 months every year, and they couldn't have provided what they did for me for another child also without her working full time. There are definite benefits!
From what you've said here, I would advise against having another. The only reason you should have another child is if YOU truly want one -regardless of the child you already have. I have no idea how old you are, but you probably have a few years to play with as well. Put it on ice -and in a year or two if you feel differently, then you can revisit the subject. There's nothing wrong with being or having an only child though!