Sudden demand avoidance and school flame-out in wonderful son

I have a wonderful super-bright eight year old who's always struggled to be in school.

He was excluded from a private pre-reception for trashing the classroom and his current mainstream school have implemented steadily-increasing accommodations for him, including an EHCP and a full-time support worker. He finds it hard to be around other (noisy) children and only got up to 90% of the time in the classroom at the end of last year. The winter seems to be worse than the summer due to common colds and other kiddie viruses making his sensory issues worse, and he has super-violent meltdowns - trashing classrooms, hitting and kicking people. He's regularly restrained by staff.

He seemed to be settling well into Year 3, but - then - he got a virus that caused him to feel faint in a morning (he never actually fainted). From then, he never seemed to recover. He's always slightly tired (not ME/CFS tired, just fatigued) and he started needing an adult to shadow him in the morning to get him through his going-to-school routine. He started having meltdowns at home over minor things, developed a fear of the dark (needing an adult to escort him upstairs in our house), and eventually couldn't be in the classroom at all without melting down repeatedly.

As of this week, he was going into the classroom through a quiet corridor before other children arrived, and completing worksheets alone in a quiet anteroom, but still melting down twice daily. He especially hates writing because, although he's meeting age-related expectations, it's not good enough for him because his maths/science are so good (at home, he watches popular science videos constantly, and has somehow managed to teach himself KS3 (Year 7) maths off YouTube).

He's been at school for two days this week, but was off today, and I tried to get him to do an emotional-awareness exercise with me and he melted down solidly for ninety minutes - running about frantically and becoming non-speaking.

Not sure what to do, to be honest. It's the most terrifying thing in the world having a son who spent 40 minutes this afternoon silently drawing diagrams of the Earth's position in the Milky Way, the variola virus and equilateral/isoceles triangles, while recovering from a non-speaking meltdown, but who now seems terrified of being taught anything.

He can't really explain the meltdowns. He's adamant that he's not anxious or upset about anything, but he seems to have zero access to his own emotional states.

 I'm pretty sure I'm undiagnosed autistic myself, but I've never seen anything like this. He seems happy and sane when not melting down. I noticed that there were a few autistic adults on here posting on the family threads. Wondered if anyone had any advice/suggestions? I've taken him to the GP to check for anything physical (obviously) - we're awaiting a blood test for anemia and he's been given some Vitamin D.