my uncle is 96 years old. other than being elderly, he has no clear medical diagnosis except complete exhaustion. he sleeps all the time, although when he's awake, he's totally cognizant and fully functioning. someone suggested he just has "failure to thrive" but i think there may be something medically wrong.
Have the Drs. treated him for depression? Failure to thrive can be equated with giving up. Loss of quality l life can trigger this and also bring on depression. I don't know nor have I seen your uncle but these are possibilities.