Does he have a bad odor from his beak? It is possible that he has canker, but there can be yellow plaques from viruses, fungus, or bacteria as well. Canker smells bad from what others have said. It is treated with Fish Zole (Metronidazole) 250mg daily for 5
-7 days.
http://www.poultrydvm.com/condition/canker
-7 days.
http://www.poultrydvm.com/condition/canker