Hi, I am Dr. Peter and would like to help.
1- Any blood in the diarrhea?
2- Besides the diarrhea any other symptoms?
In response to your question, you should feed him a bland diet of rice (75%) and boiled boneless chicken (25%). Do not give him pepto bismol or immodium. If the diarrhea does not resolve within 24 hours of changing his diet or if he becomes lethargic and vomiting have him checked by your veterinarian, it is important in a young dog to rule out a foreign body, colitis, parasites, etc. In the meantime, change his diet as above.
If you have additional questions I will be glad to answer them.