Welcome to the board. Most likely it is the high corn content in your dogs diet, although, as your dog is seven it could also be due to needing a different type of diet, meaning specifically senior designed. To help the coat be less dry and brittle, I would suggest supplementing with Omega 3. Brewers yeast is generally used as an appetite encourager in case you weren't aware of that. I would suggest having your dog checked by the vet as well as any change in coat could very well mean something else going on as well.
"For every animal that dies in a shelter, there is someone somewhere responsible for its death".