Heeelp... I have everything else and DS doesn't actually have allergies so I've got him the polyester/viscose/bombproof trousers but I reckon if it IS hot next month he will want shorts and he will find the manmade fibres too hot. He has sensory issues and will be struggling with tailored shorts anyway, and generally tries to wear as little clothing as possible.
Nowhere seems to have even cotton mix school shorts, in fact generally they are out of stock in other fabrics too (Next and Asda do have some black in age 4 - M&S are OOS - John Lewis only do navy or grey! and Sainsbury's only do sports shorts in black).
I am thinking of taking a risk on these but he is quite small for 4 1/2 so they may well be too big and fall down (he wouldn't mind if they were a bit long as most of his shorts are).