> I've seen him not being super nice to other people who were trying to have a conversation with him, not because he's not a nice person (I found him quite personable one on one), but it seems to me that he struggles to know how to behave around people who don't know how to just talk to him about things he wants to talk about.

I'd argue that while he may be nice, it's also generally considered impolite to be someone who "only talks to him about the things he wants to talk about". It's meant to be a two-way street, generally. Someone who only wants to talk about what -they- are interested in, not what their conversation partner is interested in is not being nice or polite.