Red Rash On Hole Body, Itchy. Diagnosed Psoriasis, Exzema, Scabies, Etc. Treated With Prednisone And Ultra-violet Light Not Helping

My father , an 87 year old male who is also an internist and oncologist, retired, presented with a red rash. It began in February 2013 and started on his trunk and has now spread to his entire body. He has been to 4 dermatologists who have diagnosed psoriasis, exzemsa, scabies, etc. Supposedly, a biopsy was done, but perhaps they dind't go deep enough. He's been treated with prednisone and ultra-violet light. Nothing has helped. He is constantly itching and his whole body his covered with red rash, pustules and dried patches. Can you help?