You should send your 7 year old to school in a diaper.
Your 7 year old should also never be potty trained or if he already is the 7 year old should go back to wearing diapers full time and no longer use a toilet.
If the 7 year old is wearing a diaper to school or all the time then he can just pee and poop in the diaper and not need to ask to use the restroom at school.
Nobody will know the 7 year old is wearing a diaper when he's wearing pants or shorts.
Diapers are very comfortable as well and he will learn to love the diapers if he doesn't already.
At home keep him in just a diaper or diaper and shirt.
Diapers are more comfortable and feel great when wet or poopy and they are much better than holding it and having to run to the toilet or find a toilet.
All kids should remain in diapers no matter the age.
