Regardless of whether or not there are judgements excluding school uniform from EA legislation, it’s still a perfectly reasonable thing for the op to challenge the head on.
I would also like to ask him on what scientific basis he is basing his decision on - I assume there is evidence that having hair over the collar disadvantages boys learning in a way that doesn’t disadvantage girls learning? In which case what is that mechanism and what happens when a child decides to change sex - at what point does the change happen - is it gradual or a flick of a switch? (Not that I agree that dc can change sex but given all the hoohah around it, might as well use to your advantage!).
I would also point out that, as others have said, in this day and age, it is incredibly ignorant, sexist and discriminatory to suggest that hair length has anything to do with being professional or a person’s ability to do their job. For good measure I’d point out that it means he as a head teacher is failing if he is unable to teach boys with hair that is longer than collar length, given that the boy in question is complying with all school rules about having their hair tied up for pe and science etc.
I would talk to the head about it and then if he failed to budge if you have legal insurance on your house insurance etc I would ask them to write a letter pointing out the facts about it being discriminatory and illegal to see if that helps.
I get the problems your ds has - ds is 11 and also has dyspraxia etc and struggles with hair brushing. The one thing that has made a difference is I bought him a little brush by a company called Wet Brush - very thin flexible metal ‘bristles’ that get through tangly hair but don’t hurt or pull (was £4 from tk maxx or a tenner on amazon) - I’ve also always had a small cheap brush from Home Bargains or Poundland or similar in the car door, one do each dc (and of add a load of cheap band each too) so if hair gets forgotten I’d can be done easily on the way to school. One less thing to stress about!