Many dogs have food allergies. It is far more common than most people realize. 2 of our 3 dogs currently (had 4, lost one this summer from cancer) have multiple food allergies. Some are "mild" allergies and some are "severe". We are using 2 different foods at the moment for the 3 dogs because the dogs couldn't agree to be allergic to the same things.
Our vet allergy tested our dogs. You get a report back for each dog on what their allergic to, and how severe the allergic reaction was. If it's above a certain threshold you should avoid giving your dog any food with that ingredient in it. Some food allergies can produce strange results in the individual dog.
I would recommend having your dog tested for allergies (it's a blood test sent off to a lab), so you're not feeding the dog something that's making him ill, or tired, or itchy, or "gunky" eyes, or doing funny things to his urine ph levels, or any number of other things.
Some of the things dogs can be allergic to are surprising. Some of the ingredients in dog food (I don't mean bad or harmful) like "flax seed", etc (we have a dog severely allergic to flax seed) are also surprising. A very "high quality" dog food can still have stuff in it that your particular dog is allergic to.