I used to have princess dresses and such, I also used to wear my grandmas skirts and blouses when around the house sometimes..I even had a dalmation outfit that I was quite partial to after watching the film

But my parents would never have even thought to send me out somewhere looking like that, especially school/playgroup. There is a time and a place IMO.
I have an inkling if he was turning up in a full spiderman suit, or dressed like fireman sam or something it would have been a problem too. Nothing to do with his gender.
The mother is just attention seeking. Things like this really do no good for REAL gender identity cases tbh...