[Simply put, it's the age difference that he's inferred. Unless he's certain the person is around his age, 'san'. If he is, 'chan'. It's a cutesy way of talking he's used to.

But he lets out a little huff.] Yeah, I know. I look like a limp wet noodle boy right now. And that's because I am! But...yeah. [He nods.] I met a 'Professor Ozpin' over the network. He invited me to join his classes. So. I'm gonna pick up skills there, hopefully.

[He tugs his arms to his chest, determined.]

Don't underestimate me. I'm tiny, fast, and I steal kneecaps for fun.

[Okay, that last part is obviously a lie.]