If he can write comfortably and well, it shouldn't matter how he holds the pencil. He's got to be using a lot more muscles with that grip than with a regular one!
My parents were both lefties, and they held the pencil in an ordinary grip, just with the opposite hand from all us kids. My mother did tend to curl her wrist up over the writing, but that comes from learning to write with ink from an inkwell! She also had a teacher who smacked lefties with a ruler to try and make them righties, but it didn't take.
