DLA to PIP phone interview terminated

I am nominated to act on behalf of my son (16 years old and transferring from DLA to PIP). I completed a paper form PIP application and then he was sent a face to face interview which when I phoned to request be changed. This was readily agreed to be changed to a phone assessment & it was agreed that my son did not need to be there for it. (He gets anxious and angry due to his autism and would not cope with the negative information being presented, also has his gcse’s, which are piling on stress and pressure, non of which he is really coping with). Today, the phone appointment was all set to take place but the assessor said she could not do it as my son was not there. I explained this had been agreed. She checked with her manager and said that it was wrong and that my son had to be there and must try to participate. I explained that this would lead to anger and potential aggression, but she said that there was nothing she could do. I asked what questions he would be asked and she said basically all those that were on the form - and a few more. The form took a long time to complete and it would cause distress for my 16 yr old to have his difficulties spelled out in this way. He rejects the notion of being vulnerable and thinks his physical size and strength are evidence that he is not. I am so anxious about him having to go through this assessment and the impact on us as a family that I am not sure i can go ahead with it. Please can anyone advise?

Thanks for your reply Gary.
Yes, I am my sons appointee, sorry, I used the wrong term.
A couple of things have happened since my post. After the ‘terminated phone interview’ I was sent a text inviting my son to a face to face interview (so - going backwards not forwards at this stage!). I decided to call DWP and spoke to a very helpful man there who said that IAS were wrong and that the call should have gone ahead without my son. At this point it seems that DWP are working on new guidance - but the IAS are either ignoring this guidance, don’t understand it, or don’t know about it. I was able to ask the helpful DWP man why i needed to do the phone assessment as i had completed the paper form - he looked at my form whilst we chatted and said that the answers were good and detailed and that it was because the school counsellor that he had seen had not replied to their attempts to contact them, and so it had been escalated. He also said they wanted to check about medication - i said he’s not medicated, as there are no tablets for autism. Anyway… i now have another phone interview booked and my son doesn’t need to attend, but this time I understand it is with a DWP representative not an IAS rep, so hopefully this means someone who knows about the new guidance.
Thanks!