Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry that you have been waiting for a response, but I would like to help if you'd still like an opinion. I am concerned that your fellow is getting very thin even with a good appetite.
When dogs aren't able to properly digest, absorb and use the nutrients they are taking in malnutrition follows and lethargy is the result.
When we see weight loss with an apparently normal or increased appetite we need to worry about disease processes that either don't allow him to digest and/or absorb the food he is taking (such as diabetes, liver disease, kidney disease, primary intestinal or pancreatic disease including cancers) or those that lead to a higher metabolic rate such as cancer or a chronic infection.
Parasites rarely cause an adult dog to lose significant amounts of weight unless they have several and are being fed poorly.
If he were drinking more and urinating a tremendous amount, diabetes would be likely. Sometimes early on with diabetes we only see weight loss. Even with a great diet if he cannot move the calories out of his bloodstream into tissues he won't benefit from it. Diabetes is controlled with diet and insulin therapy.
Has he had any testing done to achieve a diagnosis?
Ideally he should have a complete blood count and biochemistry profile with along with a urinalysis. I would also submit a fresh stool sample to have sent out to the laboratory for a parasite screen. These tests often give us the information that we need, or they point to where we need to look next.
If those aren't diagnostic then it is time to move on to more specialized testing. I recommend checking vitamin B levels to look for signs of intestinal disease, a test for pancreatic insufficiency called a TLI, and an abdominal ultrasound to look for signs of changes in the walls of the intestines as well as evaluating his organs.
He may need an endoscopy to collect biopsies of his intestines if they look abnormal.
I understand that he is an older fellow, and you might not want to put him through a lot of tests. But perhaps just a mini senior blood panel, and a urinalysis will help point toward a diagnosis and help us help him regain some weight and maintain weight.
In the meantime you might try feeding him puppy food or a higher calorie prescription food called Hills a/d or Iams Maximum Calorie from your veterinarian and make sure he has access to plenty of fresh, clean water.
You might also try worming him with a broad spectrum wormer called Panacur (fenbendazole).
Please let me know if you have any further questions.