lettuce cause diarrhea in dogs so I would stop feeding your dog lettuce for awhile. I feed my dog Blue Buffalo and I do not consider it to be holistic. I would stop feeding your dog fruits for awhile too then feed him one fruit at time and see if cause him to have diarrhea . This is call the elimination diet it is use to help find which food a person or pet can't eat.
If you're feeding him a lot fruits and lettuce at once that will give him diarrhea and it not a balance diet . Bananas and apple are binding so if you really want to feed your dog fruit these two fruit will be OK . But you should not let your dog eat the apple seeds or skin.