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Hello. Thanks for the question.
I would want to make sure he doesn't have Diabetes. Diabetes can lead to weight loss, change in appetite and cataract formation. It can also predispose animals to secondary infections in the GI tract, which could explain the diarrhea. I would get some geriatric bloodwork to make sure this is not the issue.
If the bloodwork is normal, then it's probably all aging issues- geriatric diabetes, arthritis and gastroenteritis. It could also be an intestinal cancer (Lymphoma). I would make sure he's on a consistent diet to rule out a dietary issue. I like to use Hill's J/D (joint disease) in these older arthritic dogs. It really helps with their joint health. It's also a bit higher in calories so it can help with the weight loss. Your vet would have to order this diet for you, as it's a prescription diet. Adding 1-2 tablespoons of canned pumpkin to the diet can also help. The extra fiber will help with the loose stools that you are seeing.
I hope this helps, let me know if you have any other questions.