If you are really and truly sure you are done and ready to leave, I wouldn't say anything until you had all your ducks in a row and everything arranged. What would be the point, if you are truly done, to give him advance notice that you plan to leave? If you aren't doing it in order to try and get him to "see the light," then there's no reason for him to know you are leaving. You say he won't try to stop you, so, just do what needs doing and leave.