I wonder.... when my son was your son's age, he was pretty emotional sometimes. We used bathtub play, and that really calmed him down. It is something to think about. The sensory input from water, is very different from eyes, and hearing.

I even did it before school when he had an episode of fear about a new teacher. Having pillow punching contests also helped, channel that tension. For a while we had an inflatable punching
toy...I don't know even know if they make these anymore!

My son was born with severe vision defects, and was considered blind for a while. So he used to get overloaded because he didn't get glasses until he was 8mos old. And still with his astigmatism, they can't correct that until the child is much older.
He didn't have a head injury, but emotional overloads in kids are pretty common and mostly due to excessive stimulation.

I often wonder how he would have turned out today, with all this emphasis on video games, etc.