Boys school trousers

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I need help with boys school trousers please! My 11 year old is currently wearing Tesco 12-13 Regular Leg trousers, but he has hit puberty and has developed a rather round bottom! The trousers fit him fine in the length and on the waist, but it suddenly looks like the seam on his bum is about to rip!
If I buy Plus Fit, they’ll be too big on his waist, won’t they?
Can anyone recommend shops and styles that will accommodate his shape please?

The Trutex ones are pretty good because they've got the half elastic back to allow more room at the rear. Their sizing chart is also good because if you pop his measurements in, it'll tell you the most accurate sizing for him. Junior Boys Sturdy Fit Trouser UK | Trutex School Uniforms

These are very good, just pick the age of the child (no need to size up), they're hard wearing too:

amzn.eu/d/dPWZJWE

I always buy marks and Spencer's. They have adjustable waists. My son has a full bottom (sorry son, that's my fault!) but they fit fine in the M&S ones....

https://www.marksandspencer.com/2-pack-boys-regular-leg-trousers/p/clp60450510?color=BLACK#intid=pidpg1pip48g2r4c1%7CprodflaggplptssCBS_2

I think the plus size ones have those elastic/button thing going on, I usually buy from Next as my son is 12 and in age 17 trousers so also been through that thickening them thinning out stage once he hit 5'11"

The plus fit ones from Asda aren’t actually that much bigger than normal. I normally order them in a couple of sizes and see what fits better.

I tried him in some M&S ones in the next size up from what he’s in now, and they were still tight on his bottom!! They also felt weirdly scratchy and cheap, not like the trousers I’ve had from there before?

I’ve ordered some plus fit half elastic ones from Asda, will report back!