I am so sorry you and your little boy is going through this, AutumnSky, especially when you have both been through a lot.
I can imagine the very conflicting emotions you are feeling and I think that is often exacerbated by simply not knowing where to start (and perhaps by simultaneously knowing that, wherever you start, you may be opening a huge can of worms and ... worse ... possibly not even going in the right direction).
But start you must and it might help you to take those first steps if you remember two very obvious and important things. One, that your little boy is not behaving this way because he's happy and, two, he is not trying to keep his behavior hidden, in fact he is overtly (and rather shockingly, for you) trying to bring it to your attention.
It is an absolute minefield trying to access the right sort of help, especially when you don't know what the problem is, and especially harder when, as you say, your DS is not compliant about going down the medical route.
But you can initially set things in motion without him, so start by doing what you know. Go to your GP without your son, and tell him/her everything you know. Also, within the same very short time span, go back to the school and tell them that you don't know what has caused this behavior, but you are very concerned about your son's wellbeing, and you would like an emergency meeting to take place between yourself, the school head/principal and the school district's educational psychologist.
Then take it all from there. Your GP will almost certainly refer to CAMHs, but also tell your GP that you would like his/her continuing support in addition to any referral ... help to make sense of things, to know what to do next, and especially support to push for further help when needed. The school/education psychologist may try to withdraw once your GP refers to CAMHs, but tell them pretty much what you tell your GP ... that you would like their continuing support in addition to any referral, etc.
Start this all in motion tomorrow, AutumnSky. Do not be willing to wait weeks for appointments with GP surgeries or school agencies. Whatever is at the bottom of all this, your DS needs your help now.
Keep talking to us. 