Should I tell this guy I have a curtain for a door?

Hey, if you take it seriously, he'll take it seriously. If you take it positively and in a jokey way, he'll take it like that. You control how he takes this. If you want a casual joke about it between him you should joke away. The more negatively you treat this the more negatively he will react. Just be positive and don't pretend it's not there when he comes.