Mental disorder?
First, if you watch the video....there’s no way that this kid is 7 years old. He’s at least nine or ten. So I think he’s got an age issue.
Second, the minute that a teacher or school authority directs you to do something or stop bad behavior, and you don’t seem to be able to take orders, and continue physical violence, then the cuffs are legit.
If I were the school, I’d have a meeting with the parents and demand the kid attend anger management classes prior to be allowed back into the school (figure a 7-day suspension minimum in this case).
If the parents try to move the kid to another school....it’ll just repeat itself again.
Maybe he was trying to earn his first tattoo?