Does your son have any siblings/ cousins or other children that he plays with regularly out of school. How does he negotiate sharing/taking turns/ not always being first with them. If he is ok with this, what causes the issue at school
I have no idea whether your son is autistic or not but you could try some interventions that help autistic children.
Have a read up about social stories and/or ask the SENDCO about them
Some social stories on line are aimed at those with learning disabilities but they can work for those without these too. Sometimes they can have an almost magical effect
Key things are to write in the first person, name feelings , say if something is ok, read it repeatedly.
So something like:
My name is Daniel and I am 6 years old. Every day the teacher ( put in name) asks us to line up to go and wash our hands before lunch. Some times I am fist in the queue to do this. I enjoy being first.
Often though another child is first in the queue. This is OK. Everyone likes a turn at being first.
When this happens i feel sad and angry. This is not nice but it is okay. Sometimes when I feel sad and angry I hit and throw things. This is not ok
Instead, if I feel sad and angry about not being first I can...( talk to ( named member of staff) about my feelings/ write about my feelings in my feelings book when I get home/ go and sit in the calm corner and take 5 deep slow breaths/ - whatever other strategies you and school work out to help him manage the disregulation) .
It may be, if he has some kind of neuro- diversity that a noisy open classroom is quite stressful for him and his window of tolerance is "used up" dealing with this. Then he just doesn't have the tolerance to cope with otter stressors - like not being first.