I have no experience with this but, somewhere in the recesses of my mind, thought Bernese Mountain dogs can get this (among other cancers) Went to a site and found this, which really does not sound like your dog's symptoms.
Disseminated - to spread
Malignant - uncontrollable or resistant to therapy
Sarcoma – a malignant tumour beginning in the connective tissue
Metastasis – the spreading of disease, especially cancer from one part of the body to another.
Hystiocytic Sarcoma (HS) was first reported in Bernese Mountain Dogs and now recognized in a number of breeds. It is a very aggressive form of cancer that is usually wide spread before any symptoms are noted. The symptoms usually include loss of appetite, severe weight loss, lethargy, weakness and general poor condition and sometimes high temperatures.
Hystiocytic Sarcoma mostly presents as a malignant neoplasia of internal organs with widespread metastasis. However, localized HS may arise in a number of primary locations including: spleen, tongue, lung, brain stem, nasal cavity, spinal cord and also can be affected are the lymph nodes, bone/marrow, liver, central nervous system, kidney, skeletal muscle, stomach and adrenal glands.
Blood tests may be normal, or may show anaemia, among many other changes. In some cases where the cancer has been detected early, it is possible to remove the cancer. It has been known for bloods to return to normal, but the disease is one that attacks many organs and it is so rapid that the dog eventually succumbs very quickly to the disease.
To date no treatment has been found to be effective against fighting this disease. There has been many studies done about this disease and many conclusions were that HC was inherited. Meaning that the dog has a much higher chance of developing this cancer, if one or both parents have had the disease.
Survival from diagnosis may vary from hours, days, perhaps weeks and very rarely months. It is truly a rapid and aggressive disease and the age of onset is generally 7 years, but can and does, occur in much younger dogs.
I'll keep looking around. This *may* be what a dog in the neighborhood has.
Histiocytic sarcoma is recognized as occurring in several dog breeds as well as in humans. In Bernese Mountain Dogs, a genetic familial susceptibility to the cancer has been observed. Other dogs that seem predisposed to histiocytic sarcoma include Flat-Coated Retrievers, Golden Retrievers and Rottweilers. Symptoms of canine histiocytic sarcoma include weight loss, low energy, limping and failure to eat. Chemotherapy has been mostly unsuccessful against this form of cancer, and most dogs succumb to the disease within months of diagnosis.
I agree with you decision vs quality of life issues. If he seems to be doing well on the pain meds, that's a good thing. At this point, I would try to make him as comfortable as I could.