"of course they can wear them if they like them."
[emphasis added]
And that's the whole point.
This whole thread sounds very much as though it is the OP that is forcing this, not her DS requesting it.
.
"...simultaneously gender critical and also obsessed with what’s appropriate for boys and girls."
If an individual child wants to go to school wearing any piece of clothing that is part of the allowed school uniform then, yes, they absolutely can wear whatever they want.
However, in this case, it appears that the child has not requested this. In this context, given that it is the OP that appears to want to force this on to her child, then it is quite right that others should perhaps point out how other children may react and what the consequences may be.