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
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.