You’re exactly right. Boys wear very little colour these days.
And clothing has become increasingly gendered. When I was a kid, young girls and boys pretty much wore the same clothes: e.g. brown cords and red polo necks. Sisters clothes were passed down to brothers and vice versa. Teenage boys and girls wore blue jeans and t-shirts, all had similar hair: long, short, dyed or spiked. In the 80s, the fella who did my hair wore skirts and big hair, just like his wife who ran the salon with him (New Romantics).
Now, so depressingly gendered.
And school uniform makes no sense on so many levels.
But I'd get him the grey shorts. Unless it’s a hill you both really want to die on.