If the relief shows on his face he doesn't realize it and he is relieved the brother isn't a frog and hasn't been eaten. So far the man seems nice enough, not dangerous. He doesn't get that feeling from him, not in the same way he's used to back home.

"Native-magic? Is your magic very different from the magic here?"

Is this even real? He's having a conversation with a stranger about magic. He's being told he has magic. He's walking around a so-called magic academy in another world so different from his own. How does he even process all of this?

Jack would know what to do.

"I'm...Sebastian...of..." He thought for a moment, unsure how to introduce himself. "Earth?" He was looking at his hands again and then realized he should probably do a proper introduction. He just couldn't bring himself to offer his hand freely to a stranger. "That's a planet. Where I'm from."