What possible reason could he have for resigning? He could trot out some tired excuse like "I'm leaving to spend more time with my family" or something, but the thing is I doubt he even has to do anything other than go to a meeting once a year or something and collect his checks, which are probably more per month than most people make in a year. Essentially he was on the board because they wanted someone of his stature there, and most likely that's pretty much all that was expected of him.