PIP tribunal query

Beginning the appeal to tribunal for my grandson re. PIP
How much information should go in the first stage. Have downloaded the appeal form but am unsure if I should put all the information that I ahve in with this ie medical reports and written eveidence that I included with the request for mandatory reassessment (10 pages of typed information relating to where I felt he should have scored points.)
Just been reading the guide about appearing before a tribunal - what should be put in a written submission and what should go in the appeal form?
Also the guide says that the panel will direct questions to the claimant not the representative. I'm worried about this as the claimant is 16 years old and has autism. Although he is not completely non verbal I think he will struggle to answer questions appropriately and will become confused and distressed. There is a strong chance that he will not speak at all!!!! I know that he can have a written appeal but as he did not have a face to face medical assessment it seems that the HP made a lot of assumptions about how he copes and how his disbilities affect him. I want him to have the best chance possible at winning his appeal but wondered how the panel will react if he doesn't answer their questionning.

Thank you

I was made his appointee by DWP when they came out to see if he could manage his own money. They decided he couldn't and his money is paid into my account. Is this the same thing, if not how do I become his legal appointee.

Many thanks