ESA face to face assessment

Hi, my son is due for An ESA face to face assessment soon, he suffers with social phobia, social anxiety disorder, depression,
Insomnia, autism & behavioral problems, my question is should I take with us any further letters or evidence in with the face to face and would they look at it? Would a letter from his Doctor confirming his illnesses and his inability to work at all help, If I Do ask his Doctor for a letter should I cut straight to the point and ask him to explain that he is incapable of any type of work or work related group and advise he should continue to be placed into the Support group?,
Only a few months ago he was asked to transfer from DLA-PIP and complete that form so this recent ESA form and now face to face has only worsened his anxiety and worry, the DLA to PIP was successful and his award increased [as well as filling in the form we did receive a phone call which I can only describe as a phone call face to face in a way but not a direct face to face], thank you for your help.

thanks Gordon, after looking at the forums a form ESA113 03/11 comes across, would this be ok for his Doctor to fill in and take with us or would the ESA people have already asked his Doctor to fill one of these in ? His Doctor did fill in the Personal Independence Payments
Claimant Request for GP comment questionnaire that you have on the forum, could we take this along to the face to face to at least show some of his comments which were very informative and useful and although this was for PIP the general comments do give a good indication of his problems regarding activities in some areas of ESA, Thank you again.