I think this is probably 'idiopathatic' diarrhea. Meaning there can't be a real diagnosis. It just happens. I would just ignore him as you clean up the mess. Don't make noises, like 'Oh its OK', or Oh dear, poor puppy. Just clean it up and go on.
My 9 year old had very soft stool this am, but made it to the pee pad. It was a mess to clean up for sure. 6-8 'spots' on the pad. These are washable pads, and they wash up beautifully.