Been following thread with interest.
Not in similar situation, but a bit saddened to see how grim people think it must be to be an only child. :(
My dp is an only child, and he's one of the most giving, loving and sharing people I know. (He was asked when he was little if he wanted a brother or sister, and he said no!) His mum's an only child too, and yet between him, his mum and his grandparents - this tiny little family of four, across three generations - I often think there's more love than in most bigger families. My dp's dad is one of three, and he and his brother and sister had a massive fall-out when their mum died - over inheritance - and have barely spoken since. Interesting.
My best friend is an only child too. Like one poster said, yes, she's going through that "ill parent burden" phase. Her dad has Parkinson's, so she's finding that tough, supporting her mum and dad.
But although I'm one of three, I'll be in the same boat some time. My elder brother lives in Australia (and doesn't plan to come home), and my younger sister, who I adore, has Down syndrome. So when my parents get older, I'll have them to look after plus my sister, who has quite high needs. So it's worth considering that having another baby could actually multiply the caring burden of our healthy firstborns, not halve it.
Research suggests that, temperamentally, only children are little different to firstborn children. The sharing/selfish/lonely thing is a bit overblown, in my opinion/experience. And I know so many families where the siblings don't get along; it's always assumed they'll play together, but they might hate each other.
I'm not sure if I want another child yet - am finding my son's toddlerhood a bit full-on! Also, I worry about another baby having special needs; I've been helping care for my sister since I was five, and feel I need a break from caring, which isn't all that likely as she and my parents get older. But I can imagine that if I wanted another, and couldn't have one, I'd be devastated, so I do feel for you on that score.
Just wanted to give a different perspective to your situation. Hope you, dh and ds find a way forward you're all happy with. :)