OP, I would in all honesty stop making threats of boarding school - it really won't help!!!
I would also get CAMHS involved again and find out more about ODD, I personally think you will find this is a main option/reason for your DS's behaviour.
You have basically described my DS1, he has had a very privilidged childhood as well etc etc and he has now been diagnosed with ODD, after many years of telling GP,child psychologists, nurses etc that I knew something was not quite right. We are waiting for the YIT -youth intervention team, to come out and visit as this is an option for finding him help and activities where he can take his frustrations etc.
I would suggest trying to spend some time researching the disorders mentioned and also I would discuss it with the school.
Your son doesn't need threats of boarding school!! he needs help and only you can get this for him you will be failing him if you don't!
It is, as I know, so easy to forget that this little boy is YOUR son who you gave birth to etc when he is in full swing, trust me I know but you have to keep reminding yourself of it because it is easy to get caught in a trap.
Please contact your GP for a referral to CAMHS because you and your family NEED them to help you ALL.