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.