This happened to me. We had a child and he has a potentially hereditary condition. My partner said he didn’t want anymore after this, he found babies and children in general harder work than he thought it would be plus the potential for any further children to have the same condition wasn’t a risk he wanted to take. Prior to this, he had wanted two children while I had always wanted a big family but accepted two as what we would have.
It was actually really tough for a couple of years and I couldn’t get over it. Eventually I told him I would end up resenting him if we didn’t have more kids so I would have to leave for both our happiness. He then changed his mind and now we have 3

now he always says he put too much thought into it as he’s such a worrier and is so glad we never left our oldest as an only child.
I know this probably makes me look like a control freak but that was my experience.