Can a UC telephone assessment be conducted with an appointee?

My adult son has just been notified about a telephone assessment and I think it'll be problematic if he has to speak to an assessor. He has issues with severe nerve pain and misalignment, hypermobility and he's also autistic and tends to "cut out" or meltdown if required to speak to a professional for any length of time. He basically mistrusts them after years of medical mismanagement and futile attempts to get them to understand. His extremely low frustration tolerance means rage is very easily triggered which, after failed appointments like this, can affect not just him but all of us for days. He also finds his pain and postural issues are triggered by trying to hold a phone or move a computer mouse plus the timing of any appointment is almost bound to be difficult as he reverse sleeps (i.e. is mostly nocturnal).

Would it be OK for me, as his mother and appointee, to speak to them on his behalf? I can probably get him to give agreement over the phone, if I hold it for him, but otherwise I'm getting very anxious about the whole thing. There's also the issue of minimal evidence from professionals as, since he can't even get downstairs, he can't attend, say, rheumatology or physio appointments without a great effort which can then set him back for weeks if it goes wrong. He also tends to reject their help as he "knows" it won't work or has tried their approach (e.g. physio exercises or gentle walking) and he reckons it's damaged him further. He's also in denial about being autistic, even after a full assessment and diagnosis, and he has significant sensory issues that can flare up unpredictably (e.g. don't give him anything in cellophane, for example, or shrink wrapped etc).

Phew! That's quite a relief!