As a social worker, when I hear that someone has no energy and sleeps a lot, I think of depression. You say that he’s not depressed and has a good sense of humor. It’s possible that even he doesn’t recognize that he is depressed or grieving over the loss of his kidneys and the changes in his life. Or it’s possible that he knows he feels sad but tries to cover it up by saying he’s fine and showing his “good sense of humor.”
Here’s an online depression screening tool from the NYU Med Center/NYU Dept of Psychiatry that he could complete by answering the questions honestly and having the tool score the test letting him know if he’s depressed. If so, he should get a psychiatric evaluation and treatment.
http://www.med.nyu.edu/psych/screens/depres.html
If he is depressed, activity/exercise can help along with medications and talk therapy. Cognitive behavioral therapy has helped people with depression. His social worker should be able to provide some counseling but if he needs more time, he/she or his nephrologist may be able to refer him to a community psychiatrist, psychologist, or social worker who has worked with people with chronic illnesses, like kidney disease.
You and he may want to review the Kidney School module on Coping with Kidney Disease (Module 5).
http://www.kidneyschool.org
Good luck!