I agree, take him to the doctor's. If its something "out of the blue" it could be a passing cold or flu. If its something that has been happening for a while it could be depression or something more.
Tell him to go to the doctors, or take him yourself.
Best of luck!
I am assuming this is a chronic problem that has not suddenly due to the flu. He needs a general physical to include bloodwork. The ER is not the place to go. This should include a hematocrit and consult with an endochronologist, whom will evaluate his blood sugar levels, testosterone and thyroid function. Insist on getting copies of all lab results. When dealing with chronic fatigue, no matter how good your GP is, a consult with the endochronologist is very important. If he does not have insurance tell the doctor ahead of time. You will find you can ask what the tests will cost and bargain over the prices. The price for "cash" tests is often much lower than what would be billed to insurance.
Thank you very much for your advise.