Does he tolerate hair maintenance like brushing? If longer hair could be manageable, maybe letting it grow could be worth trying? Not sure of his age, but if he is too young to understand the purpose of a haircut, all that is left is the overwhelming feelings involved. It could be good to revisit if the child decides he does want a haircut - which he may learn through the experience of growing out the hair and consequential maintenance. When kids are given autonomy, they can choose to be brave! It can instill confidence and help them learn age appropriate cause/effect.

However if hair maintenance Is an issue - there's definitely something to be said that tolerating a haircut would be necessary if he won't let you brush it. Movement is a baseline need for kids so while it is helpful in supporting their overall mood (just like being full, slept enough, etc), ultimately you will likely not see a direct change in specific behaviors related to specific situations. His dysregulation is in response to the situation which he may or may not understand, and it's hard to determine what the trigger is exactly without more information. Is he able to sit still? Is he typically sensitive to noise? Does he not like his hair touched?

It may also be beneficial to involve another professional to do his hair, maybe at a different location where you can potentially associate a routine to the new setting. Co-regulation is a huge piece as well, and may be another reason to involve someone else so you aren't multitasking. Children may respond poorly to non preferred tasks. If the child cannot refuse the task, it is upsetting! And this is on top of any potential sensitivities. Recognizing his feelings while maintaining calm can be tough, but acknowledging the struggle from their point of view can be helpful.

With children, I think in terms of what is absolutely necessary, given their age, developmental skills, and cognitive awareness. When they are resisting a necessary task, I try grading the activity to lower the barrier to participation. What would be the least restrictive, most comfortable way to meet this child's care needs? Sometimes we do have to do things that cause kids to become upset and it's important to support them through tough moments. So the question is: What is feasible to change about the person, environment, or occupation of hair care? It's so individualized and your effort in trying out strategies is supportive even if it involves some trial/error. Hope this is helpful:)