I had waist length hair as a child and never questioned the rule that said hair below shoulder length needed to be tied back. The same applied to my ds2's hair years later and until he got tired of managing such long hair, his was tied back in a pony tail. No drama, no need for a drama.
I happen to have an intensive dislike for very short, clippered hair on boys. It looks hideous. But that's neither here nor there, tbh because I respect those parents who opt to turn their boys into mini-skinheads.
I'm not a great fan of school uniform either since it is utter bollocks to claim it's either inclusive or has a great deal of effect on discipline but the simple fact is that if the school you send your children to has uniform rules then you have to go along with them. The alternative is to choose another school or home educate.
I realise that the OP's son has sensory issues but as The Lady Evenstar says (from her own experience) somewhere along the line you may have to go against your SN child's preferences. Also, it is almost always wrong to assume that no other child in a school has been asked to comply with a rule. Unless, of course, you are the one that sends all the letters out to parents asking for long hair to be tied back.
Clearly, OP, you have much bigger problems than merely tying back your son's hair. It looks as if you dislike where you live, dislike the school your children attend and generally seem to assume that you've been transported to an uncivilised, heathen place.Although, rather oddly, it is a place where they recognise "bloody" for the swear word that everyone else (regardless of where they live) knows it is.
I'd stop making such a fearful deal out of all this. Talk to the head teacher, tell them that you will tie your child's hair back but would like to see this rule properly enforced for both boys and girls. If it is so painful for your son to have his hair tied back then I fear that you will have to tackle his unwillingness to have his hair cut and just get on and do it.