Take a stool sample to the vet to check for parasites/worms. giardia, coccidia
Stop giving PB and other treats.
Bland diet for a few days of boiled beef or chicken and white rice (overcooked and starchy) and add a good probiotic (not yogart, not strong enough) to sooth the intestinal tract.
But first get a sample checked by the vet in case he needs meds. (take the firmest stool you can find or thickest diarrhea. Watery diarrhea is very hard to test)