I would ask now for home tuition until such time as EHCP is finalised and a suitable school identified. Put this request in writing to LA.
It would be better if absence was medically signed off, but even if it's not they should take the views of the child into account. His views are that he has lost trust in school and the teacher and wants a fresh start elsewhere. Due to his anxiety (& poss rigidity?) associated with his condition he is not likely to shift his view. Has Camhs recorded this as his views or will they?
Home tuition isn't just for children with medical needs the LA should also step in if a child would not otherwise be receiving education for a period (s19 Ed Act).
The LA will probably try and tell you it's the schools responsibility to pay for alt ed as he is still on roll but it has legal duties too where a school is not doing this.
I would ask the LA to involve the person in charge of alt education / children out of school - it should have a policy of what to do when children are out of school. You can also report it to the education welfare officer.
You can make a formal complaint to LA about lack of education under s19 (once you have alerted them and given chance to sort it out) and can take this to Local Govt Ombudsman if LA doesn't offer anything.
I don't think you need to get into a blame situation too much - you and he have lost trust and reality is he won't go back to that school, the LA need to take this situation on board and put an alternative (tuition or PRU or diff school) in place so he doesn't lose further education whatever the outcome of EHCP.
You can also ask him to be placed in special school as emergency measure without EHCP
I'd try and sit down with LA and go over interim options but read up on s.19 first. The LA should not be leaving him with no education.
If the LA believe he can go back to this school then they can put measures in place themselves eg send a TA or social worker to the house to help you get him there in morning / assess his anxiety. Or they can prosecute you for non attendance - what they can't do is just let him sit at home with no education they should be doing something.
If LA try and fob you off just make an immediate complaint thru corporate complaint process - you can usually do this on website.