So, if I understand it - your son gave him all new food then? Not a huge deal, but probably irritating his system - as you can tell. Some canned pumpkin would probably help him a bit. At dinner time, if it were me - I'd probably just feed him his old food. And then tomorrow, feed the old food with about 1/4 of the new food and proceed from there. Pepto is fine for dogs. If you think Pepto would be a benefit, give about 1/2 tsp. to him.
