If your dog is getting thin and not eating well he should have a complete veterinary exam to rule out any possible disease problems. If everything checks out then trying to get the dog to eat is the next challenge. If a dog normally eats dry food he may have decreased consumption because he has a hard time chewing the large kibble. By feeding a kibble with smaller pieces or moistening the food with water it will be easier to chew. Adding canned food or gravy to the food will make it more appealing.
Some dogs can tolerate a small amount of milk or eggs added to the food.
Homemade diets of boiled rice, potatoes, vegetables and chicken or hamburger works well with others. Some dogs prefer cat food and will eat that readily, but this is often quite high in protein and should be avoided, if possible. With an older dog that won t eat it may be time to break the rules to get him to eat. Better that he has table scraps than nothing at all. Use common sense, however, and stay away from fatty foods.