PIP face to face tomorrow for 16 year old son

Hi. My 16 year old son has his assessment for PIP with Capita in the morning. Understandably he is very anxious about the meeting and worried about what they will ask him.
I've given ihim all the information I put in the form about his condition to read through but do you have any hints and tips about what to say & what not to say. I really want to prepare him as best I can as I've read some horror stories about these assessments. He's a lovely, quiet lad and I think he's going to really struggle

Well what a farce, me and my son were both up early & ready for the appointment at 9.15 but at 9.25 we get a phone call to say it was cancelled as the HP had not turned up for work this morning !!!
I explained how unhappy i was as I had taken a day off work and asked if they could send someone else but no joy. She offered another appointment on the 28th June but I said I was unable to confirm that until I went into work on Monday and checked if I could have time off that day. The woman then gave me a phone number to ring them next week and arrange a suitable day.
Reading on the forum I gather the cancelling at the last minute is quite common but not the ringing them back bit. Has anyone else come across this ??
Sorry about the rant. Seriously peed off as my son is now really stressed out and I'd rearranged everything today around their appointment !!!