If you notice, once the driver gets back into the seat, he tries to get the seat belt to work. However, due to the design of the belt system with the gravity lock, it's difficult to accomplish.
Suggestion: what I've done in my Bronco is to use the 'I' adapter for a baby seat. This 'I' makes the belt into a lap belt. Walmart sells them at the counters.
Not this is the best thing to do, but at least it works for stock vehicle's restraint systems. If the driver secured his butt into the seat, he wouldn't have been tossed around. His torso would be a little sore due to the muscles reaction, but he would have been planted in the seat. To add, the added benefit is that you can still be moveable in the seat to lookout the window.