Eat some grass
Have you allowed him to eat some grass? Grass is good for doggies stomach when they are upset. Give a little pepto bismal for those real bad cases of upset belly.
I would start adding canned dog food to his diet. Canned foods have more calories than dry. Mix the two together.
He was checked for parasites correct? If he starts eating wood objects check for worms.
Dogs tend to eat less in the summer and more in the winter. How many times a day do you feed him? Once a day is appropriate but may need to increase to twice. Adding water to dry food can be an issue. I had to add water to my dog's food temporarily as I was dealing with a tooth issue. However, by the next day I noticed bugs/worms had formed in the food if not all eaten. My other dog's didn't particulary care for the wet food. They would leave it in the dish.
I hope he gets better soon. It just may be a sensitive stomach issue. Good luck!
Scooby, Shaggy the "Dogs", Ms. Thang the "Cat" and introducing Measley Weasle "The Ferret".