Turns out, Sigmund got so bad he had to stay at the vet's office for two days. He had an ultra sound one day and an endoscopy the next day . Good news! He does not have lymphosarcoma - he has inflammatory bowel syndrome, which Dr. Murphy suspected all along. He gets prednisone in his ears twice a day (for 8 weeks) and he eats only hypo-allergenic dry food (for the rest of his life).
There were early signs - he was only able to eat Royal Canin's gastro intestinal canned food for quite some time. Everything else made him vomit.