What about any non-food related allergies? plants, new carpet, etc? has there maybe been any recent changes in the family that could have trigger some stress? Sometimes even what we consider to be a "small thing" can mean a lot to them.
Does he get enough exercise? some dogs will pull their hair out if they are bored... what about parasites, has he been tested for any?
Dogs don't need to talk, their life is their message ~ unknown.
"The greatness of a nation and its moral progress can be judged by the way its animals are treated." ~ Mahatma Gandhi