Try to add some apple cider vinegar to his food. Touch his mouth and see if maybe he's got discomfort in his mouth. He is 9 years old. Gingivitis might be affecting him. Purina isn't the best dry food out there because its so full of corn and by products and grains and stuff, maybe he's allergic to those ingredients?
**Sorry to come off as condescending about food.. I had my dog for 14 years. He ate store foods made with corn and fillers and all that gross stuff for about 11 years before I started buying holistic. The difference on holistic was almost immediately and definitely prolonged his life!
Now I only feed holistic. It's not that much more expensive for the lower end holistic ("Holistic BLend" is available at grocery stores) and you feed less per meal!