Puppy Has Soft Stool

At his age, dry food is fine, but it needs to be a puppy formula. Also agree with all the posters saying a trip to the vet needs to happen; it could be anything from the dog not absorbing nutrients properly from being on adult food to worms to any one of a number of diseases.
If the stool's been runny more than three days, you're also risking dehydration, which can be serious.

I agree with BKLander about the pup being too young to eat adult food. Adult food doesn't have the nutrients nor is the pup ready to digest the adult dog food.

Just like human babies need baby food; puppies need puppy food. Their digestive systems aren't fully ready to handle adult dog food until 6-9 months. If he has loose stools/diarrhea get him to the vet.

He could be sick. Better call your vet and find out what is causing it. Many pups get diarrhea when they are young and need shots to get better or some kind of medication.