Outdoorsmum - I am so sorry that your DH fels like this, when you would like him to be more open to another child. that must be hard.
But honestly, I think the hard part is on you. Bellieve me, if you google only children, the only people posting about it will be people who are not happy with it, not those who are! I visit sites for parents of children with clubfoot, and it can be terrifying, because of course, the only people who post are thise seeking solutions to problems. yoou have to remember that.
My DS is an only child, and he seems very happy, stable, social, etc etc. We make sure he has lots of contact with cousins, friends, and we make sure he lives the life of a child, not annexed to all our adult activities (what am I talking about? what adult activities??). I tend to think he will continue to be happy in himself, and happy with who he is. And I think that is because we are happy with our choice.
I know your choice would be different, but it is your choice that has been impeded, not your DS's. You may be transferring your anxieties about your own need to have another child into worrying about DS.
I am very sorry that your DH is so adamant, but you don't need to turn that into guilt over DS.