This is a toughie. I suspect your dh desperately wants another child and so is using every excuse possible to change your mind, whilst you don't.
There is no right or wrong here. I was one of 6 and my dh one of 7. It was a chaotic household and there is no way I'd have that many. We have 2 children and it's nice. They get on well and play well together. But I know other siblings don't behave quite so well, such as their cousins (another boy and girl) who fight like cat and dog.
As for your ds being lonely, well as you say if you surround him with friends and cousins then you can do something about that. It's true that he won't have to make as many compromises or share. He'll never be short of attention and will be used to having things his own way. He may wish he had siblings as he gets older, he may not.
But a far bigger problem for you is not how your ds will cope, your ds will cope just fine no matter what you decide to do. But as for you two? If you have a baby for your dh you may well end up resenting him and how would that second child cope knowing that you wanted to stop at just one? If you don't have another, your dh will surely resent you for not giving him the extra child he wants.
Have a talk to him. Is he gunning for a girl? How would he feel if you had another boy? What if you tried but couldn't have any more? What if you got pregnant and there turned out to be something wrong with the baby, what would you do then?
Life doesn't always turn out the way we want it to. Shit happens at times and whilst you may change your mind, you might not even be able to conceive again. Or you might and you may have twins!
He cannot use your son as a tool in his arguments. Your ds is just fine and will always be just fine either as an only child or as a brother of another. There is no guarantee that having another child will improve his life one iota. As as I said, life can be shit at times and he may hate a sibling, they may fight like cat and dog, or simply have absolutely nothing in common.
This decision needs to be taken with just the two of you and your marriage to consider. One of you will have to compromise but if it will lead to a lifetime of resentment then you may have to accept that this could well be a deal-breaker. And that would have an effect on your ds. So do take the time to consider this and talk, talk and talk some more.