Getting Assessed

I am trying to speed up the diagnosis for my adult 23-year-old son. It has taken 10 years to get to the point where he agreed to be assessed. We were only referred to Autism services in Bristol a couple of months ago and the current waiting list is over 2 years for appointments.   Does anyone know where I can find details of professionals, I can pay which are acceptable? He is about to become embroiled in the judicial system and we just don't have 2 years to wait.  I spoke to BASS in Bristol and they were unable to recommend anyone as they are part of the NHS.  Thanks