Waiting times for assessment

Hi. I just joined this as a parent of an apparently autistic son who had a lot of support at school even without a diagnosis but this has dropped off since he left. We requested a diagnosis when he was 17, I've seen other people say how long it takes, I know it's normal except that he is now 23 (nearly 24) and the Notts (NESS) waiting list just seems to be getting longer (they told me that since July 2021 they've had to take on ADHD assessments as well and so the list is stretching out. My GP has told me to try using 'Right to Choose' but I am very confused by this, they sent me a link to some providers but none of them are local and the ones that I called up said they weren't accepting Right to choose patients any more. Does anyone have any experience of this? I've tried to get advice from the GP and am getting nowhere, I'm at a loss.