given his age it's not cancer.

Did your Vet send the stomach tumor off to Pathology to find out the type of cancer? If it's gastric Adenocarcinoma then it's very difficult to treat due to the metastasis. If you are located in the U.S. you might want to contact the Animal Cancer Institute to see if an anti-angiogenic drug trial is open that you could get into. If it were to turn out to be gastric Lymphoma, then you could follow up with standard Lymphoma chemotherapy.