<human>: Can you give me a shortened version of the text?

A guy I'm casually seeing has a very close relationship with a 17 year old girl. They're not related, she's his landlord's  daughter but he says he sees her as a younger cousin or sister. He says that he feels like part of the family, but he only ever talks about her and not about the rest of the family. She's all over social media about him, to me it's painfully obvious she at least has a huge crush on him.

In the past couple of months, he's taken her to see fireworks with her family, to dinner (alone) and a movie (alone), and to a baseball game (alone), and those are just the things I know about.  His ex gf who he had been friends with quit talking to him because of their relationship, and he's really sensitive about it... but it's really starting to weird me out, too. Am I being irrational? How do I voice my concerns, if I do? He tends to like people who like him and forgets that he's in his thirties and not in his mid-20s anymore.
<bot>: He's best friends with a kid half his age and, truthfully, I'm a little threatened, but mostly concerned about both of their well-beings.