Yep, his system is not liking something in that food. Royal Canin really is not a very good food (lots of fillers) are in it. Look for a grain free chicken free food to start with and switch him over a few good ones are Earthborn, Taste of the Wild, Acana, Fromm, Orejin.... you can place him on a bland diet (boiled meat a rice) to see if he will eat.
My first bully was allergic to rice... we did not know it till he was about 2 yrs old, but he was the same way -- barely ate and was slow to do so when he did. Once we got the rice out of his diet, he went back to eating normally.