Hi Kathryn, diarrhea in a puppy can be caused by many things, it could just be his new surroundings, the car ride home all could have made him nervous or anxious and he just needs to settle in his new home. It can be caused by food intolerances, but if he didn't have it at the breeders then it's probably not food related. Did you call and ask the breeder if he or any of the other puppies had diarrhea? It could also be a virus, a parasite like worms, or an illness common to puppies that he may have picked up while at the breeders. When I picked up my two guys from the breeders, we had 4 days to take them to a vet for a health check in order for the guarantee to be valid. They checked them over for any genetic defects, checked their respiratory and heart, listened to their stomachs, checked their ears, eyes and mouth, and also did a stool culture to rule out any viruses or parasites. I would take him to the vet and have him do a puppy exam and also take a stool sample so they can check it for any viruses or parasites. It's probably just his new surroundings, the excitement and also him being away from his litter mates that have caused the diarrhea, but it doesn't hurt to have him checked out, and if it is something he can be treated right away. I hope he's okay and the diarrhea clears up soon. Please keep us posted on how he's doing.