My 9 year old German Shepherd has went from 90 lbs to 45 lbs, he has refused to eat for 3 weeks.?

Our vet X-rayed his stomach, he then had an ultlasound his liver & spleen are slightly enlarged. His blood work a little off, all could be explained by not eating. He just did a small pooh, kindda greenish and slimy loooking. He was vomiting in the beginning but isn't now because of anti-nausea medicine. $700.00 later, he's still starving to death. Losing faith in my vet. Any ideas? Won't even eat prime rib :(