When my DD was subjected to a toxic school experience in which she was frustrated because her needs weren't being met, and she was spending all day trying to be well-behaved despite her frustration, that led to explosive and violent behaviors at home... with the exception that hers was inwardly directed (hitting head on wall, blaming self) rather than outwardly directed. With a different personality, I could easily see her going the other way. That's how I did it at that age.
One thing that stands out to me is that your DS is doing a lot of things on his own. Is that coming at a social cost? My DD, for example, was given time to do math on the computer in K when the rest of the class was doing a group activity with the teacher on the promethium board. DD is extremely social, so she was caught between doing something interesting, and doing something with other children. She was forced to choose between two of her fundamental needs.
In our DD's case, the situation began when she began attending school, the behaviors started gradually escalating the longer she was there, and our investigation revealed the many ways that we, with our unique insight into our DD's personality, was being harmed by her daily school experience. Once we pulled her out and homeschooled, we began to see a reduction in the negative behaviors. So yes, in our case we're 100% sure it was all school-related.
So my advice would be, if the situation only began when he started school, and it's been gradually escalating, then it's time to do a thorough investigation of what he's experiencing in his school day. I'd look at all factors... how much work he's doing and what kind of fit... how much time he has to himself, and what he's allowed to do while waiting... how the relationship is working with the teacher... how he's getting along with other students...
