The guy's approach was wrong.
He wasn't hassling the cop for not wearing a seat belt for his own safety. He was hassling him because he thought the cop was breaking the law. He made it clear he was afraid of cops and he had been "assaulted" on several occasions.
Also he was so dumb as to think that the cop might want to get out of the car in the middle of the road to talk to him.
I would assault him if I was a cop.

He might have got a different result if he had expressed concern for the cop's safety. Mention a campaign for cops to wear seat belts because the greatest cause of injury to cops is vehicle accidents without seat belts.