If the cop had taken the reasonable step to listen for a second to what the guy was trying to say, he would not have been walking back to his car.
If indeed the cop ended up having no option, he certainly should have warned the driver that he was going to taser him - which he did not.
The cop is a bully, and is clearly wrong.
The man clearly saw the officer draw his taser and point it at him. If he didn't know that he was at risk of being tasered based on that, he should probably be required to wear a helmet at all times.

I just want to see what law it is that you were citing where cops are supposed to warn you before they tase you.