My dog has diarrhea off and on. Could it be a food intolerance? What food should I try?

My papillon has diarrhea off and on, and I think he might have a food intolerance. I currently feed him Wellness small breed adult, and the first 5 ingredients are Deboned Turkey, Chicken Meal, Salmon Meal, Oatmeal, Ground Brown Rice. I'm thinking maybe a food with only one protein source would be the place to start. My cousin recommended Diamond Natural, and they have a lamb and rice formula for small breeds that I may try, what are some other foods I should look at? Should I avoid chicken and stick to lamb, or is the turkey or salmon a more likely culprit?