Exactly. I keep my chickens and ducks together, but they were integrated AFTER they already knew who was a duck and who was a chicken. Although I don't have any drakes or roosters, so for me it doesn't matter too much.
He probably imprinted on the chickens, therefore he probably actually believes he IS a chicken. Maybe give him zero access to the chickens and see if nature takes over. If he still has chicken hens available, of course that's what he'll choose. Once he starts breeding ducks, then see about...