He gets to make a family and chose his family.
The idea that he will be alone isn't true. He could have a cousin - only for something to happen to them.
DS is an only GC on my side. He has cousins on his Dad's side. But he sees the older two maybe once or twice a year as they live the other side of the country. The younger two live closer, but are still very young and COVID has made it difficult.
I have four cousins. Two live in Canada. One lives in NZ. One lives in the Highlands of Scotland. I havent seen any in a good few years.
My mum didnt really know any of her cousins - she moved around a lot as a child, and I certainly never met any - her mum was one of THIRTEEN. Her dad was one of three - one died as a child. The other changed his name and disappeared as part of a fraud meaning she lost contact on that side of the family.
My Dad father was a twin. He also had three other siblings. He didn't know his cousins. His dad's twin died abroad. My dad found his long lost cousin about 10 years ago.
You assume he would have close relationships with his cousins. It's enough of a push for siblings to have that relationship.
My family aren't close. I could go back several more generations in my family and show similar patterns of the family not being close. If we get together it's generally great but it rarely happens.
I don't have the desire for more tbh.