Hi Bonkerz
I've just been skim-reading through this and your situation sounds very similar to ours with DS who's now 9 with a dx of AS. We knew J had behavoural issues outside the norm from about 2-3 years, and I strongly believed he had ADHD. In Reception he was observed in school by a psychiatrist and I later found out that the Head had withheld information that would have led to full assessment - as a result it was left there with a 'wait and see' outcome. They didn't even get as far as Connors forms.
By Y1 things were so far out of hand that I insisted on him being assessed at CAMHS. Although the paed told me she was 99% convinced he had AS and his ADOS suggested he should definitely be placed on the spectrum, he didn't tick all the boxes so the outcome was inconclusive. They wouldn't give a dx of PDD-NOS as an interim measure to ensure appropriate support was in place - he was either normal or not and in this case, not. LIke you, we were given insanely patronising advice on how to manage him (the naughty step, sigh), which like you we'd done to death so we knew he needed more. So school continued to not support him and his behaviour got worse.
We went for a second opinion assessment and this came up almost a year later, when J was 7 and a half (5 years after we'd first suspected something was 'wrong'). They dx'd in five hours, HFA/AS with no uncertainty whatsoever.
During this time we'd treated J as though autistic, following methods we'd learned through NAS Help courses and seminars and what we'd read in books and on the internet. As a result J has made amazing progress and is doing very well. However, although many of his autistic issues are being managed he still has ADHD symptoms and we're approaching CAMHS again to look into this further. Unfortunately, we've effectively cut off our own noses because by doing all this work with J his problems are much less obvious than they used to be, so it will probably not be recognised.
Anyway, my reason for posting was to urge you to go for a second opinion. You know your child well and it's clear that there are issues that need to be supported. You need that dx for him (whatever it turns out to be) so go for it.