It would definitely help to get more evidence to back up what you are saying ie letters and reports from professionals.
Particularly if you could get Head Master to state how much help and support he needs to be able to participate in all activities in school (even if its just frequent reminders, frequent reassurance to keep him on track)
There is a big difference between being able to do PE in a structured, familiar environment. Than walking down a road, which is not familiar, with cars etc.
Same for care componment, how much help and support he needs for bodily functions, eating etc.
Questions they are likely to ask will be based around what they have stated in their letter.
You will have to prove that your ds is not similar to another child of the same age without a disability. Give examples of how your ds's needs go above and beyond that of a 'normal' child.
I should imagine they will ask you about school and how he manages to do these things he cant do at home, in school ie if he needs constant supervision at home, why doesnt he need constant supervision in school.
Good luck