Worried about UC telephone assessment and lack of sufficient evidence

We've finally received an appointment for a telephone assessment, in which I will act as appointee for my adult son and speak on his behalf. I'm worried though. The appointment letter asks that we bring or refer to any additional evidence and the fact is that we already submitted what little we had. Due to previous bad experiences with the NHS and other services, my son has refused to engage with them for some time. When he did attend, the appointments exacerbated his nerve pain, caused severe autistic meltdowns, and took so long to recover from that they just weren't worth it. Except, I now think, for gather evidence for the DWP. Should I see his GP to pave the way or try to get a letter, or would this just be a waste of time at this late stage?

Right-o. Will do. Thanks.

Well, I sent a letter to the GP so that his medical records would be updated in our own words as I couldn't make a GP appointment in time.

Today, however, the telephone assessment was delayed due to the previous appointment relating to "a lot of conditions". Then, when it eventually came through, it turned out that they'd just noticed that I requested a recording and they didn't have the equipment for that. So it has to be rearranged again.

I also tried to progress his PIP claim as I put in the request mid January and was told the form would be in the post. Apparently, and even though they'd previously refused to even send me the form because their system didn't show my confirmed appointeeship, there's still an issue with this and they referred to "discrepancies on the account". They tried to get me to contact UC to arrange a home visit to confirm appointeeship but I explained that I'd already been through this process, received confirmation and, after the initial difficulties, checked with UC that it had gone through. So now they're asking UC for that information themselves.

What could possibly go wrong?