Last episode on disc 3, season 4, Roscoe definitely says, " Good boy!" And in the same episode, Luke is curled up with Flash in Roscoe's patrol car and when he exits says, "Good girl!"
So there you have it! Flash was called a boy and a girl in the same episode! But in keeping in the spirit of things, one could surmise that that cluttered brain of Roscoe was a might confused about the sex of his own dog, since he seemed to go back and forth between episodes, and maybe, just maybe, he had confused everyone else in Hazard County about his dog's sex!