What you describe is horribly familiar. I don't understand why the system disbelieves what parents say, but it does - at great cost to the mental health of children and their parents. My son has finally received his ASD diagnosis - and he is 14. He has been seen by countless psychologists and been under the care of a psychiatrist for the past two years and none of them would diagnose him. Like you, I felt as if I was going mad.
Both CAMHS and the schools are equally useless, in our experience, I'm afraid. We never had a diagnosis for ASD from CAMHS, in spite of asking for reassessments three times.
When I saw a paediatrician privately (Dr Daphne Keen, a specialist in neurodevelopmental disorders), she believed everything we said. It was such a relief and it showed how used we had become to having to justify our opinions all the time.
If you can afford it, going private is sometimes the only way to get a diagnosis. Then people have to start taking you seriously (although, sadly, even that doesn't convince some of them ).
Have confidence in your instincts. You know something isn't right. I had the same instincts about my son and they have all been proved right, but it took years. You know your son better than anyone and the people who are undermining you probably have a stereotypical idea of what ASD children look like.
Oh, and definitely keep a behaviour diary about your son. And one about all the phone calls and appointments with professionals. It's invaluable.