I really wouldn’t wait to buy uniform, you will find by August it’s hard to come by and often sold out, especially with M&S current challenges.
If the policy is grey shorts why not just buy those. Most children like to fit in and not stand out, they are robust and hide the inevitable stains! Many boys still choose to wear grey shorts all year around at primary school rather than switching to trousers. My son and his friends definitely did.
Hope your son settles in fast when he starts.