Are you changing the food frequently? There are lot of reasons this could happen but it is best to rule out any disease or infection, like mentioned above. But once you have a diagnosis you can give your dog steamed rice and cottage cheese as these are easy on the stomach and contain carbs and protein. If his bill of health is clean you might look at what kind of water he is drinking, is it fresh? Does he drink out of mud puddles? Does he eat things he should not like feces? Cat litter? His toys? Is he drinking out of a toilet treated with bleach?
