PIP reassessment mobility score

Hello all , I'm new here.

My son is 18 , he has a diagnosis of autism ( aspergers) since 2017, prior to that he has not been in school since aged 13 . He also has diagnosis of anxiety and depression with pseudo -hallucinations. 

He has not been out of the house alone since 2013, he can only leave the house if he has someone with him. He can make trips to the psychiatrist and psychologist plus other small trips as long as he has someone with him. On the Pip he scored 4 points on the mobility part  ie needs prompting to make journey, but I am now beginning to think this is wrong even with prompting he cannot make a journey alone, he has had PIP since 2015 and I only just realised following the court case regarding the mobility scores can include mental health difficulties, I just got the letter from PIP saying they have assessed the claim following the court case but he still only gets the 4 points , how can this be right if he cannot go out alone due to extreme anxiety?

I am going to do a letter for reconsideration but wondered if anyone had thoughts as to whether I am right in my view? 

He gets the standard rate for the daily activites but nothing for mobility and when he was on DLA he got the standard mobility rate. 

Thanks all