Do the symptoms of protein losing enteropathy fit?
Ellie has the same issue but isn't too lethargic and isn't losing weight uncontrollably. It is really frustrating. It may just be IBD but her vet said that because we might have to go to prednisolone if nothing starts working soon (I still need to switch her diet...), she will push us to get an intestinal biopsy done because in rare cases IBD type activity can be caused by cancer .
Have you tried probiotics?
What is the protein source in his diet? Fish?