Can Appointee answer at health assessment?

Just joined today. My 21yr old son has MLD and ASD. We have the face to face health assessment next week (has been in support group indefinitely since age 16)
He is anxious about attending, wants to just put headphones on!
Can I answer on his behalf as I am his recognised appointee?
Does he have to attend, can I attend on his behalf?
Thank you

Thank you for your response.
My son is 21, and we have spent many years building up his confidence and telling him he can do things - which he sees as him doing independently, and not recognising the amount of support we have had to input to get him there
eg - A train journey to a familiar place - he says he can do. I say I've looked up the times, ordered his ticket and sent him screen shots of the train times.
Sequencing personal care - he says he can do. An example last week was I got him up (he failed to set alarm), he came down un showered in jeans with holes (inappropriate for days activity) so had to change, he failed to eat breakfast because he didn't have time, he was about to walk out of the door in a T shirt and it was sub-zero outside, he put coat on and missed bus from stop opposite the house! These are examples of his learning disability affecting his day to day activities - but he would say he can get up, dressed and leave with no probems!
We do have a psychological assessment that says his processing of verbal communication is in the bottom 0.1 percentile of the population, and he has severe difficulties in this area (along with other areas) Therefore he will really struggle with answering questions in an interview situation - he will talk a and say things but they won't be terribly accurate!
Therefore, in an ESA assessment (and have just had to apply for PIP too) he will talk about all the positives, without any recognition of the support he requires on a day to day basis to achieve these things.
I also have a real problem with emphasising his disabilities in front of him, when we've spent so long building his confidence........