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I am sorry to hear about Duffy's diarrhea. There can be several causes of diarrhea in puppies, so I recommend having a fecal exam performed y your veterinarian and deworming him as parasites are a common cause of diarrhea in puppies.
If you have just switched him to the Science Diet then it may be that he needs more time to adjust and any diet change should be made slowly by mixing the old food and the new food to allow them to adjust int he different percentages of fat, protein carbohydrates and fiber. If this is the same food that he has been eating all along and he has been dewormed appropriately, then trying another brand may be helpful in relieving his diarrhea. A few of the brands that I a familiar with are Solid Gold, Natural Balance, Nature's Recipe, EVO (Innova), Eukanuba, Iams, Purina (Proplan), Diamond, Blue Buffalo and Fromm to name a few. Some of these are available only in speciality stores and others are available in grocery stores. Because he may do well on one type of food and not another, there is no way to predict how he will do until it is fed. Most or all of these should be available as samples either from the company directly or in pet stores that carry them. I recommend talking to your local pet store and seeing what they carry of these brands and trying a sample or two to see if Duffy will like it. Once you find one that he likes, you can purchase a bigger bag. Be sure to keep it in an dry, airtight container as it can spoil or turn moldy and don't purchase more than you Duffy will be able to eat in 3-4 weeks as once opened dog food can loose its flavor and palatability.
Does this help answer your question about Duffy? If you need more information or clarification please do not hesitate to let me know.