The face he gives back is Pure Teenager because that's exactly what he is, alien or not. It says, in no uncertain terms, that that is a really silly idea.

"I don't normally go around using my... stuff much. But here, no one cares, so it seemed like I could... I dunno. Enjoy it a little."

Like-

"And, uh, well, I was watching because, I, uh... I tried to help someone once. And it didn't go so well. So I thought maybe I could pick up some tricks."