As far as I can discern he got into an disagreement with some classmates about boys wearing dresses. His classmates' assertion was that a boy wearing girls clothing was super weird and my son was like "no it's not, it's not a thing. Who cares." And has decided to wear a dress to school in order to show that it serves the same functional purpose for boys as it does for girls.

Obviously I am fine with it. And we talked about how he shouldn't be doing it as a 'joke' because that could be seen as making fun of trans/non-binary folks. He's not worried about being bullied or anything like that. I think I'm equal parts proud and amused. It's really cool that he has no worries about being perceived as unmasculine and is willing to show solidarity like that.