My advice would be to accept that he doesn’t want a second child, or decide that having another child is important enough to you that you leave the relationship. What I absolutely wouldn’t do is have another child with someone who has made it clear they don’t want one.
My SIL had a second child after her husband had been very clear that he didn’t want any more children. She says the pregnancy was accidental, although I’ve always had my doubts about how accidental it really was. It caused huge problems in their marriage.
They’re still together and their youngest daughter is now six, but his relationship with her is noticeably different from the relationship he has with their eldest, who was very much planned and wanted by both of them. He provides for both girls equally, they’re included in the same holidays and family life, etc., but emotionally he simply isn’t as close to the younger one. It’s incredibly sad to see because obviously none of that is her fault.
I’m not saying every man would react like that, or even that most would, but I do think it shows the risk of deliberately having a child with someone who has already told you they don’t want that child.
And I also wouldn’t have another baby purely because you’re worried about your son being alone in the future. A sibling doesn’t guarantee a lifelong friend. I know plenty of adult siblings who barely speak or are completely no contact, so in practical terms they may as well be only children.
With an age gap of almost ten years, there’s also no guarantee they’d ever be particularly close anyway. They’d spend a lot of their childhoods and some adult life at completely different stages of life. I think another child needs to be wanted for themselves, rather than being brought into the family to provide a sibling for the child you already have.