Boxers, are a breed with fairly sensitive digestive tract. They tend to do better on Chicken based foods (as opposed to beef, or lamb) to help with "flatulence issues" Boxers are so famous for.
In general, store bought foods (Iams, Eukanuba, Alpo, Old Roy, Purina, Pedigree, Kibbles N bits, Beneful, to name a handful) are poor quality and do not agree very well in general with Boxers sensitive systems. It would be better suited to buy an upgraded variety of foods for best outcome and better stool output.
Better foods benefit for multiple reasons. A dog with proper nutrition is healthier, less behavior issues, and smaller stool volume (which we all like :) Some higher quality dog foods would be (California Natural, Royal Canin, Ennova, Wellness, Chicken Soup for the Soul, and believe it or not, the Kirkland (Costco) brands of food (to name a few).
Supplements are always good. There are many suppliments on the market, but a well rounded dog food doesn't really need it. You can add things like Salmon oil or canola oil (in small amounts) for glossier coat. Cottage cheese or plain yogurt is good for active cultures. Probiotics are great to aid in digestion.
It is also good to let your pup have some (raw) chicken pieces from time to time. Cooked chicken bones are very dangerous for dogs (they splinter and pose a puncture risk) but raw chicken bones crush and pulverize when chewed, as well as the enzymes in the raw chicken help with digestion and better BREATH!
Raw food diet for dogs, or home made food, is the best most nutritious option, but not all of us can afford the time or money to do so. If you are interested in finding out more about raw or home made diets for dogs, click the link below for more info.