Something is off. 9 year olds are young I don't see an eating problem until around 13 or 14, unless something is wrong.
I'd do the vet visit and also spice up the food. Try adding different things and see if he eats. If he has been on it a long time, he may be bored. Will he do ok on another food? Even CA Natural but the other protein source?
Many people think dogs are picky when in fact it's all in the nose. Where you and I smell dog food, they smell every ingredient! So, if one ingredient is off, they won't eat.
Call the company and ask. Could be they have a different supplier for one ingredient.