Jimjams - it's now a long time since I worked in the Health Service, so I am really rusty about best routes to follow.
Is there anyone at the Health Authority who has shown ANY sympathy to your situation? If so, why not invite them out to coffee to ask THEIR advice as to the best way forward. Not asking them to DO anything for you - just some pointers as to who you should be contacting/what you should be doing.
I don't know much about autism, but did read a book called "For the love of Anne" over 20 years ago, which was how Anne's parents managed to encourage, bully, teach and love Anne out of her "other" world of autism enough that she could at least start to function in the "normal" world.
Where is the research evidence that "it is impossible to treat" autism? It may be impossible to CURE it, but there are lots of things that can't be cured and people still get palliative therapies to make "it" easier to live with.
Provide them with the evidence of what he HAS achieved with supervision/instruction. Present him as a challenge which, with the right support, could actually "make" one of the SALT's reputations! If they think he is impossible to treat, think of the brownie points they would get if they could present his as a success story through their efforts (swallow your pride on that one!)
Turn it around and massage their egos!