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You have to be clear with your grandson that this is unacceptable behavior in your house. Tell him he either starts using the bathroom to pee or he will have to move out. Let him know this has to start immediately. If he is having some problem which is causing this behavior tell him that he needs to seek help for this immediately or move out. For him to continue peeing in cups, bowls, whatever is just rude and disrespectful. If he chooses not to change this behavior you do not need to put up with it. Good luck and be strong.