I have a similar aged (nearly 26 yr old) son with autism, anxiety etc, and it is REALLY really difficult to get any support.
My son sounds less able than yours (was special schooled etc) but does have a little local job and lives with us and could not live independently without support.
The problem lies in being not 'disabled enough' to 'qualify' and not being able enough, or too velnerable, to live independently. My son has been on some mythical waiting listsince he was 20, and frankly I don't think it will ever happen.
Friends with similar adult children have found the same. Some (wealthier ones) have clubbed together to buy a property and place 4 adults with SN together, and then fight for funding for support/ use UC and HB to help pay for support. We arne't in that position unfortunately.
Has he had a needs assessment? I'm guessing that is the first step (adult SW team)... we did years ago but have dropped off the radar.
Does he have MH support/consultants/ anyone who he sees who may be able to back up requests for assessment and support?
I wish I had something helpful or positive to suggest, but frankly it's shit and you have to fight for ANYTHING.