Maybe he just needs a re-direction. When he mouths your hand, DO NOT take him outside. Instead, ask him, "do you need to go out?" in a higher pitched voice that lets him know that you want a response from him. When he gives you another indication other than mouthing, say, "Good Boy", and take him out. Keep working at this untill he gives you a vocal response, which is what you want from him when he needs to go out. But totally ignore the mouthing for a going out response. And if he mouths at other times, close his mouth with your hand and keep saying no until he gets the idea, then walk away from him to re-direct his thoughts. Habits are hard to break but they can be broken with repetitive training. Good luck.