Advice on benefits for our adult son

I`m already registered with you but can`t remember details to sign in so I`ve used Facebook!       I understand you are talking about the disabled and problems with employment! I live with my adult son who has high-functioning autism! He hasn`t had employment for many years! HE, I feel is discriminated against by employers and the DWP alike. He only gets JSA and 73.10 per week , no disability benefits although a few years ago he was in receipt of DLA for his autism! It was stopped when PIP came in and we had to apply for this one. I have to fill all his forms in and explain letters that arrive for him because he lacks understanding of many things which I know is part of his autism! I can`t seem to get ESA permanently! He`s had knee problems for the past couple of years (a torn meniscus in his left knee) which he had an operation early May this year!
He has to have investigations soon for the right knee as this is now causing problems!
He was once a gardener for the local council when we lived in London but now cannot do this type of work because it`s too physical! He is nearly 51 and I don`t know where to go from here. He was turned down for PIP, has to have a health assessment test next week and he needs to stay on ESA to cope! Any advice out there?