Has he/you gone through the Functional Behavior Analysis (FBA) route yet? That was one thing that did help my son because everyone sat down and thought about what the triggers were, what time of the day he usually got angry, etc. Then they wrote the Behavior Intervention Plan (BIP).
One important piece is noticing the rise in frustration before the anger takes over. Having a teacher that can recognize the physical signs was very important in our case. difficult child's physical signs of building frustration were very/are clear. So that was one piece.
The other was finding some calming techniques for him. Here are some things that worked for us. There was a fish tank in the classroom that he enjoyed. He would be given time to go over and watch or feed the fish for a while. He was given time to go over the reading area (beanbags and carpet) - noone could really see him there - having other children look at him would increase the anger. Verbally giving him the option of walking down to the office to sit for awhile or the nurse's office also helped. It made him feel he was in control rather than others controlling him. That was a big piece.
Also therapy played a big role. His therapist helped him identify how he was feeling inside when his anger was building. Some tactile things he could do without involving others emerged as a help. Pulling out his stress ball at his desk helped. He had the bottom piece of a piece of velcro on his desk that he could rub (his teacher came up with that one) helped; also rolling a pencil in his two hands helpled.
Naturally, it really depends on the child and what is comforting or relaxing for them. Looking at a book in the reading area of the classroom was a great tool. In third grade, his teacher had a baggie near her desk that contained some markers, a water bottle, paper and some crackers. She told difficult child if he was feeling frustrated, he could go up to her desk, sit down and get his bag. Then he could sit there and have a snack and chill but he had to draw or write some words about why he was upset.
For my difficult child, it was a matter of giving him back the control that seemed to help the most. Now that he is entering 6th grade, it's by no means a perfect solution. He still has his molments of noncompliance when angry. He still is easily frustrated. But some of those early interventions in second and third grade helped him identify his own feelings and certainly lessened the severity and the frequency of his outbursts.
Hope some of this was helpful to you.
Sharon