Helene
Welcome to the forum, you might want to have a look at the following FAQ which explain where everything is
Welcome to Benefits and Work If this is for PIP then there is clear guidance on how assessors should deal with a claimant with an appointee, they will try and direct their questioning to your son, but should accept your answering without problem.
I'm afraid if it is for ESA then the guidance is less clear although the same rules should apply.
Both the PIP and ESA Claim guides have a section covering the assessment and the questions likely to be asked.
Gordon