Well, that is an interesting response. Sans can feel his expression shifting with interest, and even though this guy can't see him, he fights to force it back into his usual neutrality. Just because one person can't see his weird human face doesn't mean he shouldn't keep working on figuring out how to keep it from showing what he's thinking.

"Oh yeah? Seems like you have a handle on the fashion, though." Or maybe suits just don't change much.