I am so sorry he is so ill. Have they done the GI panel (blood sample) that is sent out to Texas A&M?
He could have a pancreatic insufficiency or other condition that is picked up by this test:
Gastrointestinal Laboratory - Texas A&M Veterinary Medicine & Biomedical Sciences
Click on Assays to read about it.
In the meantime, if you have not already tried it, ask them if he could take Mirtazapine for his appetite. Sometimes it works wonders.
Will keep Murphy in my thoughts and prayers. Please keep us posted.