It's not at all uncommon for dogs to experience digestive upset like this when they are eating a new food. (A good reason to swap food gradually.) It can cause an increase in bacteria found naturally in the stomach and intestine, resulting in the diarreah you are seeing with him, even sometimes bloody stools as well. I've used Pro-Pectalin before when mine had similar issues, more than once. If I recall correctly, it did take a day or so for it to kick in and do what it was supposed to. Until he gets over his tummy issues, feed him a very bland diet of boiled chicken and rice and just hang in there. He should be good as new in a week or so. Just hang in there and smother him with kisses in the meantime.
