Is it autistic burnout or anxiety disorder

My 14 year old son is diagnosed ADHD and ASD.  He is currently experiencing EBSNA and I am trying to ascertain whether the underlying cause of his EBSNA is autistic burnout or anxiety so I can work out the best way to proceed.

He has always struggled on and off with anxiety around school. In KS1, he would try to hold on to the car and often, once in the playground, would be dragged off me (please don't judge, wish I knew then what I know now). Changed school for yr 4 and things improved but then lockdown happened. Returned for end of yr 5 and yr 6, wasn't too bad although there were odd instances of anxiety and also of feeling that things were unjust.

Coped with yr 7 quite well with odd episodes of feeling unwell first thing which got better mid morning (once pressure of school had been removed). This increased during year 8 to the point where it was happening weekly on average. He received support from the wellbeing team and this helped him to get through year 8. However, things got steadily worse this year (yr 9). He found one of his teachers very authoritarian (teacher is ex military) and found these lessons particularly difficult. School agreed that he could work in the support centre for these lessons and any others he found difficult due to behaviour of other students (this has always been something he has struggled with). This helped a bit until the week after Christmas. His teacher left the room during a lesson and the class behaved awfully - his words were utter mayhem. The next day, on the way to school, my son suddenly turned round and ran back to the car park where I had dropped him off sobbing.  He said if I had already left he would have sat there all day waiting for me.  My son has not been into a lesson since. Attempts to get him into school would see freeze or flight, shaking and physical feelings such as nausea, headaches snd achy muscles.  After 2 weeks off, school put in place a reduced timetable for him to work in the support centre. He was able to do this most days (not Tuesdays which was the day of the incident). However, this became more and more difficult for him. He went from doing school work in the support centre to reading a book the whole time he was in there; went from mornings to just the lunchhour to socialise with 1 friend. I started to see bits of my happy son returning during the Easter holidays (a vety low demand 2 weeks). He managed 2 lunchtimes back after Easter by which point he was back to the withdrawn child he had been before Easter. He has not been in school since. Says he doesn't know why, he just can't do it. Over the past 5 months, he has become a lot more clingy, there has been some regression in behaviours to preschool years and he becomes withdrawn and subdued when talking about school. There have been occasions when he has punched himself and banged his head against the wall. He has talked about feeling he has no future. He says he is broken, he wishes I could fix him etc.

I feel like I'm at a crossroads and don't know which way to go.  Do I try to focus on the anxiety and look at anxiety support programmes or do I treat as burnout and give him more time to recover.

School are being very suppotive.  GP and CAMHS don't want to know.