Thank you for the information.
Being an older dog losing weight and drinking quite a bit of water, there are certainly a few things to consider with Benji.
Kidney failure. Many dogs will have a slight to increased consumption of water with a chronic weight loss. Many times we have no idea their kidneys are beginning to fail because the signs are so subtle. There are certainly supportive things you can do for Benji, such as change in diet, to help his kidneys.
Diabetes and other endocrine disease. Being an older dog that consumes alot of water and has weight loss, diabetes is certainly a concern. In dogs, diabetes is insulin dependent. The body stops making enough insulin so their blood glucose goes out of control. A simple blood glucose test at your vet's can determine if his blood glucose is normal or not. There are also a few other endocrine diseases such as Cushing's and Addison's disease that can affect older dogs causing weight loss and increased water consumption.
Liver disease. Liver disease can definitely cause a chronic weight loss. When the liver doesn;t function properly, the body cannot break down and process the food we eat, so can affect weight as well as water consumption.
Cancer. Certainly Benji is an older dog and is at a greater risk of developing a tumor. Should he have a tumor in his stomach or infiltrated in the lining of his intestines, this can affect nutrient absorption and can cause a chronic weight loss with little to no other signs.
I would definitely recommend a checkup with your vet to have Benji examined. I would also recommend having some bloodwork done to check out all his internal organs to ensure they are functioning properly. In the meantime, make sure Benji is on a good senior dog diet-make sure to follow the directions for how much on the side of the bag. You can supplement it with some canned food or low fat gravy.