There are many causes for a puppy having diarrhea. First of all how old is the puppy? If you got him from I pet store a would definitely get him checked out by a vet. Coccidia (an internal parasite) will cause diarrhea in puppies and kittens. It is also hard to treat depending on the circumstances. You need to rule out any type of intestinal parasites first. Back to the age of the puppy, if he is still weaning then yeah his food needs to be mixed so that he may properly chew and digest the food until he is able to handle the larger chunks. Since he is a new puppy in a new environment that could cause lots of stress on the little one even though he may seem happy and playful, which would also cause diarrhea. Different types of puppy food could also give a puppy diarrhea if it is a poor quality food w/ the first ingredient being corn or corn starch that could upset his stomach. Also, switching foods from what the pet store was feeding to what your feeding will also upset his fragile still growing stomach. He should be fed a diet high in protein and fats. With my puppies I added raw eggs and yogurt to their diet occassionally or if I saw that they were having diarrhea. Yogurt especially helps a young puppy's upset stomach. Make sure that he is not vomiting or having excessive diarrhea, that is not a good sign and should be taken into the vet immediately. Excessive diarrhea will cause dehydration and signs of lethargy, vomitus, and diarrhea can all be signs to a sick puppy aka parvo. ESPECIALLY since most pet stores are nothing more thank heartless puppy mills who do not properly take care of their pets. The best advice I can give you is take your new pet in to see your Vet. There he can give the initial innoculations, test for parasites, answer any questions you might have, and start you on your way to a healthy puppy. Good luck on the new addition!