My father in law has had three bypasses and now has a pacemaker and defib. some days he seems so lethargic and so tired. he is 91?

Various causes: Some medicines can cause this which may be at too high of a dose, such as a beta-blocker. He may be developing low thyroid (hypothyroidism). He may be anemic (low red blood cells). His pacemaker may not be functioning properly and his heartrate may then be too low. His heart function may not be good anymore. He is 91, which also may play a role in his underlying energy state.