You're right about having to make sure they eat. And I've found it isn't enough to see someone preparing a meal and just assume everyone is going to eat; I make sure everyone sits down at the table for a meal. Another thing I think works well for exhaustion is when I put them to bed for the night (or they voluntarily retire), I don't go back and check on them until the next day. They always get up and start doing things when I go back after they've gone to bed. But if your guy doesn't respond to any of that, I'd go to the doc for a check-up. Sometimes when they have an infection they show exhaustion as a symptom.

Hope he does okay!