That's a nice food. Dry or canned? Not that it matters a ton - but there is a bit of difference. Any treats that you're giving, new/recently? Has he been on the Merricks a while? Depending on those things - I mean, you could try to move to a food with less ingredients. You could browse around some of the 5-star canned foods here and see what I mean (the 6-star canned are mostly single-sourced):
http://www.dogfoodanalysis.com/dog_food_reviews/