What does “have something in common” mean?
Great question. To have something "in common" is used to talk about two or more things or people, that have the same characteristics or interests. Here he's being sarcastic saying it because he doesn't really believe that he and the little boy have any shared characteristics or interests. Ex: We have a lot in common because we grew up together. Ex: We don't have anything in common. We're completely different people.