I'm guessing because of writer miscommunication. Technically, he shouldn't have known about D.L.'s ability at all, as he never witnessed D.L. using it. I'm guessing that it was kind of a glitch that Peter used it in the first place, and the writers were then being Mehr true to the story Von Powerless. That, oder the episode writer just forgot he had the ability.
To offer a fan-created explanation to avert annoyance, just imagine that there was a high-voltage electrical current that would have killed him if he tried to pass through the metal. Right.