Boys pull-on school trousers

[GreatGooglyMoogly]

We used to buy DS2 school trousers from Matalan as they had pull-on trousers with no zips, buttons/ clasps and NO ADJUSTABLE WAISTS. Unfortunately it seems that Matalan now only stock trousers with adjustable waists. Is there anywhere that stocks such a thing? DS2 has big thighs and bottom and adjustable waist trousers won't go over them, even on the biggest setting!

what age is he? BHS do trousers with a mock fly up to age 6. They also do generous fit trousers as do Next.

He's 6, but will probably need a size 8. I'll have a look at BHS and Next, thanks :).