autistic adult son

hi I'm new here

My Adult son has been recently diagnosed with Autism and ADHD. Could anyone advise me on how I can be sure to be involved with his care when he visits his doctor/community mental health team , psychiatrist. He was misdiagnosed with a mental health condition some 17 years ago, because he did not understand the questions, the system and as he was over 18 we were not with him. . He is very happy for me to accompany him to his CPA.s, however we are not informed when they are . He was sectioned a year ago , for a few days , (he was not ill, but had  meltdown as later diagnosed) and the hospital would not give me any information about him. He was very angry he was in hospital because he  did not know why he was there. He is in 24/7care. his is an Appointee  of the local council FPA, which handles his financial affairs

My BIG question is - is there a process\ form \letter I can provide to ensure his father or  myself or sister get to register that we are his advocate ? is there a legal 0ption?   This is really important as he does not process information properly, and agrees to anything, any med, hence his initial mis-diagnoses.   thank you for any advice