I agree with with having fun throwing, but a couple of things could be incorporated at this age without a lot of work. They are very fundamental ideas.
Though the third vid is abbreviated, I like the finish position much better. He looks to have been over his front leg and his follow through took the pitching arm to the front leg/knee. His release in the first two vids were too high. I'm betting the release in the third was much further in front of his body- a good thing.
The other thing is that he needs to work on is the glove hand. It should extend out in front of him and then come back to the chest as the throwing hand comes forward.
His landing foot is pointed at the plate which is good and his stride is nice and long. If he's stepping directly at the plate the chances that he will throw the ball over is greatly increased.
Again, as mentioned above- let him have fun. Make a game of what you are trying to get him to change.