I think you've posted here for opinions, and that's all they are; we haven't met your brother, so can only go on the (minimal) information you've given.
So, just to throw an opinion into the ring, I agree with Goblinchild.
Also, perhaps he likes the ones who leave? There are lots of women who are a bit like that; they like the chaps who play hard-to-get, and it's unfortunate, because often said chaps aren't playing at all; they're just not that interested. And I suppose there must be equal numbers of men like that too. It's a kind of fetish, really.
Fwiw, b-i-l is a bit like this. He's 45.
As Trillian said, by the time someone has reached the middle part of their life, something of a revealing narrative is demonstrated. The are standing with a path of actions, paved with the choices they have made, behind them. And each choice is, in a sense, who they are. so the path reveals who they are, rather than having to try and read signs of some secret, inner, "true" being, that life has forced into cruel obscurity.
Obviously, people do change. You hear stories about chaps suddenly settling down in their mid-years. But it would be a major change.
Agree, though, if he doesn't like the thought of children particularly much, it would be completely odd to start dating women with children, and would just result in some poor woman posting on here about the chap she's dating, and likes, who seems extremely un-bonded with her offspring.