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I suggest that you take the puppy by the vet for a stool sample and deworming medication. Diarrhea is often a product of intestinal parasites.
If you suspect that he has eaten something that may have caused the diarrhea, you can withhold his regular food, treats and table scraps and offer a bland diet. Boiled chicken and rice or boiled hamburger and rice works well. Feed only this. When the diarrhea abates, you can start to add in his regular food in small amounts until you phase out the rice mixture.
Let me know if you still have questions.