Give him proper exercise, this will increase his appetite and keep him fit. You can also feed him with digestive supplements to stimulate his appetite. Do not mix any thing with the food this will make him fuzzy. Your dog will eat only when some thing is mixed with the food. This is why it is recommended not to mix any thing with the food. if you want to feed him some thing else you can feed it in between the meals but not before an hour of the meal. It will be good if your dog eat premium brand dog food such as Royal Canin. For few days do not offer him treats and other food to eat. Only provide small quantities of food which you want him to eat and then with days increase the quantity as recommended on the pack. Make sure he is not getting the food from other sources like your family members offers him biscuits.