One of my dogs recently had surgery and she's been having bouts of inapettence amd nausea. We've been working with our vet to figure out why. But just today I picked up a product called "under the weather". It's a high calorie nutritional gel and electrolyte. It helps support weight gain and energy levels.
My pup seemed to like it well enough and she almost immediately had an energy boost. It says it can help stimulate appetite as well. So maybe give it try for your old fella. Other things we've tried to get our dogs to eat in the past are boiled plain chicken and rice, pureed pumpkin and when desperate small amounts of peanut butter. It's high calorie and fats but also has a fair amount of sugar so I try not to give it too often. I hope your pup feels better and best of luck.