In the story Little for as Long, he keeps leaking. He is enjoying being little so much that he doesn’t care about the diaper anymore and it started as a punishment for bad grades. Realistically putting him in a thicker medical diaper that has 4 tabs would make his diaper more pronounced like the pamper on Noah is. Being 3.5feet tall it would return him to a waddling stage and if he did get the sissy punishment there wouldn’t be pants to hide the crinkle from the all plastic diaper. I could see him staying regressed and being home schooled, growing up to being able to be adult at times for work but still dependent on diapers because of uti he got during the punishment or just because psychologically he needs them. Another route could be he becomes a programmer and works from home or a similar job and his mom hooks him up with a mommy.
Not sure your direction for the story so just wanted to generalize so you can insert your own creativity. Very well written, and seriously I could see stories like that and others being sold digitally on Amazon.