Pull up trousers

[AndWhat]

Have a late August boy. He's 4 10 days before starting school , have tried his uniform on and he's having a meltdown!!
He's used to wearing track bottoms for nursery, doesn't entertain jeans or chino type trousers anytime!
Managed to blackmail him into wearing the trousers but he has no chance of going to the toilet independently in them. Anyone got a recommendation for pull up type trousers to help??

Does anyone know if any stores do these trousers above 6-7 size?

There is a website called sturdy kids that does them up to about 15, not sure how big at the waist or hey are though.