Despite the obvious entertainment of seeing Monster Truck take a tumble (without getting hurt), there are some really interesting parts to this clip.
Just as the chair starts to tip - it doesn't look like that was any clue that things were going to go this way, but you turned to check on him - as if you knew you needed to.
And the look he gave you on the way down way - was not so much Dad help! as - I've been told not to do this before because the chair will fall over, and now it is and Dad is right there watching me! I'm the Monster Truck - gotta love me!
Also your reaction to him going over, assessed the situation and determined that no harm would be done so, you let him go. And then didn't berate him for doing something wrong. Many fathers would have.
My dad would have checked to make sure I hadn't damaged the chair, and then told me to sit on the ground until I was old enough to handle a chair (I'm still waiting. sitting on the floor as I type this)
I'm glad no-one was hurt and you can all laugh at it, and I'm sure this video will come back to haunt him on his 21st birthday, wedding etc etc etc hehe.