He's wrong. The keep right law says nothing about '...except if you're already speeding.' He's just being a dick and driving aggressively, if the right lane is clear and he could move over. If he's actually passing somebody slower than him, and and the guy behind him just wants to go faster, then he is in the right to stay in the passing lane until he safely finishes the maneuver.
He'll eventually find out the hard way when he road-hogs a cop in an unmarked car, though.