I did some web searching, and I found this link, it seems pretty informative. Maybe this will help you. From what I gather, it is a form of Mal-nutrition, so, I would add some good, multi-vitamins and minerals to his diet, and also feed many small meals a day loaded with dense calories, such as smoothies, milk shakes, anything he might crave. Also, high protein like meats, certain veggies and dairy products. I hope this, along with the link will be of some help.
http://www.mssm.edu/msjournal/65/07_tougher.pdf
I was hoping for an answer to my question...?. What causes it and is there any rehabilitation for one diagnosed with it? There is practically nothing useful to me when I search for it on the web. thank you, sincerely aleen stone