DLA to PIP 16 year son with Autism

Hello my son has just turned 16 and is in the process from changing his DLA to PIP. He has Autism and goes to a residential special school in Somerset, we live in Essex, named on his EHCP and paid for by Essex LEA. He was in mainstream school till Oct 2015, missed six months and so has been held back a year. The LEA are funding him an extra year and we are working with them to extend a further 2 so he can do his A levels there. I filled out the PIP2 and sent it together with the original EHCP from a year ago and a resent review. Also enclosed were various psychiatric reports re his condition. I put on the form that having to attend a Face 2 Face assessment would have a catastrophic affect on him. When he hit his low point 18 months ago and we took him out of school he was suicidal as fell of a cliff at mainstream school feeling he would never get a job or achieve anything in life. Up to that point he had always been a good student. I called  the DWP a couple of weeks after submission and they confirmed they would request a paper based assessment. When it went to ATOS again I requested one but when I rang for a update they advised they have had no request. A "nurse" called me on behalf of the DWP and went through my son's condition in detail and also called his school (i have a good relationship with them). I called the nurse the next day because the health care professional from CAHMS had not been contacted (she very kindly said she would send me a letter to support our case). The nurse was uninterested and said she didn't have enough data for a paper based assessment. I said the chances of my son attending an assessment were slim as he has missed many appointments and when they are home based he has hidden in the toilet. More importantly is the affect this would have on his wellbeing. It was obvious that the nurse had no understanding of his condition and I expect that any assessor would also be limited. There are no physical signs to his condition so relying on whether he has eye contact etc is patronising. We are a low income family and rely on his DLA to support us when he has meltdowns, his dietry needs and respite for his younger siblings caught in the crossfire of his condition. I wont though put him at the risk and its looking like i will have to let the Government win and keep the pittance they give him.