HT is sometimes the SENCo/IncO. Have you spoken to the Area IncO at the LEA?
I would also suggest private Ed Psych/ Psych assessment and return to CAMHS to push for more intervention.
If you are seeing gp anyway, ask for referral to Community Paediatrician for full developmental assessment.
It's very difficult finding your way around special needs services (whether for EBD/ AS/ any other additional need, but the further you get into the system, the more avenues there are for you to try - if you have the perseverance to get on with it.
Ask the school if the HT is the SENCo. If the HT is not the SENCo, ask to meet with him/ her to discuss the CAMHS strategies that were put in place.
Many children with behavioural issues respond well to 1-1 support in the classroom. This, however, costs money, so isn't offered on a plate. You have to figt for it. Ask the SENCo/ HT if this is an option. Ask the Area IncO if this is an option (particularly if CAMHS strategies have been implemented and failed. Do you have copies of any behavioural plans/ contracts that the school have been working on with your ds? You need to be keeping these together in a file so that you can access them and pass them on to other professionals.
Have school come up with any IEPs etc? (sometimes these are used for behaviour targets etc, sometimes just behaviour plans/ contracts). Keep everything. They will be useful for further assessments.
Sometimes if school are dragging their heels, it may be helpful to ask other parents to go in and complain that their own child's education is being affected. This worked particularly well in one instance with dd1's class (also 8yo at the time). Essentially the class's education was being disrupted on a regular (sometimes daily) basis as the emergency plan when the dc kicked off was to evacuate the class to the school hall, and contain the child until he could be talked down. He would regularly throw chairs and tables/ scissors and attack the teacher - the evacuation plan was the safest way to minimise the danger to the other children. With parental intervention, the LEA agreed to provide funding for 1-1 supports, which helped enormously - the dc was able to learn some coping strategies, as well as recognising triggers etc.
It's tough being the parent of a child who doesn't quite manage to integrate, for whatever reason, but you do have to investigate all avenues before opting out.
no-one at a boarding school (unless you do manage to get a place at a specialist school) is going to do this stuff for you - any additional needs stuff will get added to your fees, or you will be asked to arranged private assessments yourself and pay.
good luck.