I'm new to this forum and have been reading all the posts in this category with a lot of interest. Your description of your son would be identical to what I would have written about mine except my DS is 7 years old. He's currently undergoing an evaluation by psych, but it feels like it is taking a long time and in the meantime I dread every time my phone rings worrying it's the school calling about DS's latest outburst. So I've been getting my hands on whatever books I can to try to help him on my own until psych has completed their eval. So far what has helped the most is setting up a reward system. The Kazdin Method for Parenting the Defiant Child has been helpful. We still need to get to the underlying cause, but in the meantime it's helped with the outbursts. My DS really wanted to stop his behavior, but he couldn't stop himself in the moment and would just get carried off and "forget" all the anger management skills we would talk about. But having a reward system in place seems to be an aid in helping him remember and stop himself before getting swept away with negative emotions.
Hopefully we'll get some recommendations soon. We're trying to decide if we stay at his current school next year as I suspect a significant part of his behavior is from being bored at the material even though his class is supposed to differentiate for reading and math (which it really doesn't unfortunately), but if we enroll him in a school with a more rigorous curriculum will he decompensate because there's too much material and his disorganization will derail him. So it's a very stressful time for us as well. Hopefully your evaluation and ours will shed some light soon. You are definitely not alone.