I am also wondering if something else is going on here. I seem to remember some of your back story but not all of it because of my memory issues...sorry.
GFG is your bio child right? I believe so. I am not convinced that ADHD and ODD is what is going on here. While the PDD is going to complicate things, I think this has to be put somewhat on the back burner for now. His violence is completely out of control and at his age this is going to be a huge issue for him in the very near future. I am assuming that his PDD isnt so noticeable that it would get him an insanity plea immediately. Most of the real world is going to expect him to follow real world rules.
When was the last time he had a psychiatric review or neuropsych testing? Im wondering if he could be bipolar or even something else all together. Temper Disregulation disorder, explosive disorder, something. There are different medications for these things. I realize he doesnt want to discuss this but he does have some form of currency that will motivate him to attend these meetings. You just have to figure out what it is.