The bile is an indicator of an empty stomach. I would go with a better quality food than Purina. Get a food that has no meat by products, corn, wheat or soy. There's a number of high quality foods out there. Does he get any wet food? Try a little yogurt. Mixing a table spoon of canned pumpkin with wet food will firm up the stool. How's his coat? Does he have any doggy odors? How about the condition of his teeth? Dirty or clean?