I'm sorry that you're going through such a difficult time.
The actual regulations say:
(5) A person falls within subsection (3)(a) of [F2section 73] of the Act (severely mentally impaired) if he suffers from a state of arrested development or incomplete physical development of the brain, which results in severe impairment of intelligence and social functioning.
(6) A person falls within subsection (3)(b) of [F2section 73] of the Act (severe behavioural problems) if he exhibits disruptive behaviour which–
(a)is extreme,
(b)regularly requires another person to intervene and physically restrain him in order to prevent him causing physical injury to himself or another, or damage to property, and
(c)is so unpredictable that he requires another person to be present and watching over him whenever he is awake.
[F4(7) For the purposes of section 73(1)(d) of the Act, a person who is able to walk is to be taken not to satisfy the condition of being so severely disabled physically or mentally that he cannot take advantage of the faculty out of doors without guidance or supervision from another person most of the time if he does not take advantage of the faculty in such circumstances because of fear or anxiety.
(8) Paragraph (7) shall not apply where the fear or anxiety is—
(a)a symptom of a mental disability; and
(b)so severe as to prevent the person from taking advantage of the faculty in such circumstances.]
So he has to satisfy seven criteria to fulfil the requirements, which is why so many people fall down in their claims. It isn't that they necessarily don't qualify, but they don't evidence that they can tick all 7 boxes.
So: State of arrested development or incomplete physical development of the brain - Autism
Severe impairment of intelligence: Functional intelligence is impaired due to his autism and mental health condition - you can cite recent history.
impairment of Social functioning: Requires supervision during all waking hours and 1:1 tuition for own safety.
Subsection (3)(b): extreme behaviour, requiring physical intervention (this can be as little as a hand on his arm, by the way), and unpredictable (give e.g toilet - hitting own nose, etc.,)
Make it clear that he does need and want to go out with supervision, so as to satisfy the clause 8.