Does he have any health issue going on? That would be my first question. If he is honestly too tired that often, maybe he isn't getting good sleep (overweight, sleep apnea?) or has some other physical issue.
Is there a lot of stress or anxiety on him right now? How is your relationship otherwise? Sometimes financial or family stress translates to feeling worn out or low sex drive.
Finally, have you suggested to him when he is too tired that you will be on top, or do most of the "work"?
PS - I've been married for 10+ years. Occasionally (but rarely) I am legitimately too tired. Sometimes even then, if my wife initiates enough, or just tries to climb on top anyway, I am motivated to respond.
