You haven’t failed your son. None of this was your choice or your fault. It’s was simply the hand your family was dealt.
I can offer a difference perspective, maybe. I was an only child, again not by choice, because my mum suffered repeated miscarriages. Then, surprisingly, she had a pregnancy that stuck. My sibling was born when I was 10, in year 5.
It’s a big age gap. In many way we were both only children living under the same roof. We have always been at massively different life stages. It’s very difficult to be close in those circumstances. I don’t really remember him being a teen because I’d moved out by then and we didn’t really speak or see each other.
As adults were physically and emotionally quite distant. Maybe because he’s male 😉 but I wonder if things would have been different if we’d been closer in age. I mean, we don’t even share the same family memories because he didn’t exist for most of my childhood. We were born in totally different decades!
This is a long winded way of saying that, if you could wave a magic wand and give your child a sibling tomorrow, it wouldn’t be all sunshine and rainbows. Sibling relationships aren’t a guarantee of love and companionship even when born close together. I love my brother but it’s not a typical sibling relationship by any means.