mainstream schools have little to no resources to help your son. The PRU have specialist staff.
What your son needs is positive input to learn to identify and manage / mitigate his triggers, so that he can build his own resilience and self control - not have everyone making sure that he never faces anything that could set him off. This takes good one to one input from people who understand his needs and have experience in doing this. The PRU is set up for this. Its not a dumping ground, it is a, hopefully, short term stop gap to help your child to be prepared to go back into mainstream or specialised unit, or, if they are much much older, to enable them to achieve their exams and prepare for 6th form.
my daughter wasnt excluded from school - she was a school refuser because of PDA. But although the PRU wasnt suitable for her, there were many in there who were flousishing.
In addition there is the case of playing the system. You cant just reject things without trying them, not if you are wanting them to cough up for something else. You cannot say the PRU is unsuitable unless you have tried it, similarly the specialist schools.
Do you know where you DO want him to go? Because it would seem that his current school is not capable of delivering not willing to take him back. That does not bode well for a future relationship with the school. Do you have an idea for secondary which, top be honest, is the more crucial education.
I ended up withdrawing my child and she was home educated. It was not something I wanted to do, nor that successful. she has aother health issues that impacted on her ability to be in school full time as well, and the EHCP is vaguely helpful now in college.
as an aside, why did you move him from his previous school?