I am concerned that your fellow is getting thin even with a good appetite, mood changes and increased eye debris.
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, primary intestinal or pancreatic disease including cancers) or those that lead to a higher metabolic rate such as cancer.
He may not be himself because he feels weak, or has increased waste products in his body affecting his brain.
Increased eye matter may be because whatever is making him lose weight is suppressing his immune system or simply that his body is too weak to properly maintain eye health as it is focusing resources on core organs.
In an older dog the more common diseases we see include diabetes (if he were drinking more and urinating a tremendous amount too), or unfortunately cancer. Sometimes early on with diabetes we only see weight loss.
Diabetes is controlled with diet and insulin therapy.
Depending upon the type of cancer it may be curable with surgery, and/or need chemotherapy.
Has he had any testing done to achieve a diagnosis?
Ideally he should have a complete blood count and biochemistry profile, as well as a urinalysis. These tests often give us the information that we need.
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.
In the meantime you might try feeding him 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.
Please let me know if you have any further questions.