
First is he losing weight in the face of a normal to increased appetite, or is he not eating well and
losing weight? There are many causes of
weight loss
in older cats.
For example there is Diabetes, Hyperthyroidism, (overactive thyroid), and Kidney Disease, just to name a few. I would first recommend a physical exam from your veterinarian. Based on that physical exam, hopefully your veterinarian can get an idea as to weather that tumor is a problem or not.