Researcher here.
The toilet explanation is rubbish because many men wear "skirts" (sarongs, kilts) and women wear trousers (shalwar kamiz) in many societies.
I think the real reason is that society thinks men should wear trousers and women skirts - even though we don't all do that - and school uniform is much more old fashioned than clothes for other occasions. Think about the things you'd never say to a girl (ooh you can't hang upside down on the monkey bars, oh you can't play football, oh don't go out in the wet) outside of school, that school uniform says to them. Skirts are impractical for monkey bars, t-bar shoes are useless for football and your socks get wet.
I would get at his reasoning. He is young enough that he may still think this will turn him into a girl. Even if he knows that boys and girls have different bodies, this doesn't necessarily mean he will fully understand.
If it's just to be different, has he tried it. If his sister/girls in his class say "it's more comfy" they may not really know why themselves. If he thinks he'll get something glittery because he hasn't thought it through, or thinks school uniform is free, or that getting loads of new uniform isn't un-eco-friendly, a quick hint to that effect should help.
If you ARE prepared to buy new and he seems to understand the implications, I'd get him one pair/skirt to try - and have a FIRM conversation with school about why this is.
My DD wore trousers in YR/Y1 when all the other girls were wearing trousers but the uniform has now changed to trousers for everyone. She got her brother's hand me downs and didn't question it. If she had asked for skirts, I'd have gone with the "save the planet" line topped up with "but you like the monkey bars" but would possibly have got her some to try if she had thought it through.
My DS liked sparkly dress up skirts in nursery like all the other boys did, I have never heard of a nursery aged boy teased for that.
Both of them fell into the "if you put on a tiara you're a girl, if you cut your hair you're a boy"' school of early childhood cognitive thought. But children understand at an earlier age that THEY haven't changed sex than they understand that OTHER children haven't changed sex
How interesting, and helpful.
Thank you for coming on. Is it your video where little children are presented with a woman doll and a male doll, based on their clothes, and when the male doll is given the woman’s handbag the child thinks the I’ll is now a woman?
I don’t mean to derail OPs thread, and this is not related to her DS, but as you are here I just wanted to ask in light of what you have said: Do you agree with Helen Joyce, talking about Drag Queen Story Hour, that it is very easy to get a little child to think the man they are looking at is really a woman, and that also, given magical thinking at that age, very easy for them to get the idea they can change sex?