What is a good food for a 16 month bullmastiff with dry skin?

My 16 month bull mastiff has dry skin but a beautiful coat. I am considering changing his food, he is currently on Kirkland's Premium Chicken and Rice. I have tried other more expensive foods, Innova Blue Buffalo, Canidae, Evo etc and they all either gave him terrible gas, loose stools or both! I made the changes slowly mixing old food with new. The only problem I have is his dry skin while on Kirkland. His stools are fine, his weight perfect, 120 and he likes the food. Perhaps there is a supplement? Shampoo? Thank you!