The Lucky Underpants (!?)

Hello everyone, first time post as a parent of a delightful, inquisitive and fun loving boy who has recently turned five and is currently non-verbal. He has no formal diagnosis as of yet but is fortunate to have SEN support at school. Like many parents, we were anxious about the structure of routine, tasks and uniform as we are generally easy going and give our boy choice and space to express himself. 

What we hadn't anticipated was that things like uniform, instead of rejecting it, he seems to love. It's great that he is happy to get into uniform but unlike the weekends or his free time after school, his ideas on uniform appear to be very fixed. Non uniform days/Mufti/Fundraisers - they're not something that sits comfortably. Fine, no big deal. Our boy was late to mastering the toilet and had managed to refine the art of going in the holidays prior to school. New people and new environments meant he found going to the toilet a challenge. Initially he tried to hold off all day, but obviously that was unsustainable, so there were some accidents early on. 

Thanks to the support of his teacher and learning assistant, he soon mastered using the toilet with minimal supervision, however the outcome was that he only wants to wear one of three identical print briefs that came in a multipack. Attempts to coerce him into broadening his choices and explaining that underpants are not in the uniform policy don't seem to work - any pants are acceptable outside of school but inside, he's fixed to this one design and it's impossible to dress him in anything else...

We are pleading the case enthusiastically, as they are now getting a size too small and the manufacturer doesn't make that design any more - otherwise i would have about a hundred pairs lined up to substitute in. In absence of his reasoning and explanation, I am thinking that he views these pants as 'lucky' and something familial that gives him comfort. We have sought to use well loved characters, other plain pants, and even sets that have a similiar design and aesthetic - but not exact. Wondering if anyone else had this issue and please, feel free to drop any ideas!