If I had a 7 to 13 year old boy getting diaper punished I would place him some Pampers Swaddlers size 8 or Pampers Baby Dry size 7 which are both babyish.
I would make him wear just the diaper around the house or a diaper and T Shirt.
He would not be allowed to ask for a diaper change and I was check and change his diaper as needed.
I would give him plenty of water and other liquids from a baby bottle to make him pee a lot and so he will fill his diaper up.
I would also make him sleep in a crib, play with baby toys and use a pacifier.
He would also not be allowed to dress himself or feed himself and I would feed him myself with a spoon while he sat in a highchair.
He would also be permanently back in diapers until he at least turned 18 and no use of the toilets allowed.
He would have to use the diapers for both poop and pee.