To be fair, Ash never used a Pokedex for Koffing in the original series, so this could be more of a case of simply registering it to his Unova Pokedex. Granted, I didn't even see this episode (nor do I have any intention of doing so), so I have no idea if he really did forget, or simply wants to add another entry to his Pokedex. Regardless, I still think he's a moron (though slightly more tolerable in the Kalos arc).

Im sure he gets a pokedex every region and has to do it again.