That's a traditional festivity. It doesn't mean he "always dressed as a girl".
There are a bunch of photos from his childhood, and none in girls dresses. As Asuka pointed out, that's a headscarf, and the boy next to him wears the same thing. Plus Japanese wedding dresses look entirely different
Asuka, Google search gives a bunch of old threads for certain keyword searches, but sometimes I also do just click through the board to read older stuff.
