Just to stir the pot, I have a Lab and I feed him Purina with some raw carrots and Missing Link. I also give him lots of exercise. My last dog lived for 17 years (maybe it would have been longer with better food).
The way that I look at it is that I don't eat high quality, organic food, and I seem to be in good health, so I think that I can feed my dog a lower quality food, also.
However, I am making an assumption that Purina dry food is not unhealthy... just not as healthy as the better quality foods. If he had skin problems or allergies, I would change his food until his symptoms disappeared.