I would say keep him away from from food and water, for a few hours, to let his stomach settle...it could just be that he got into something outside, and it didn't agree with him. From what you say, it sounds like "explosive diarrhea" but if it is still the same tomorrow, you should probably call the vet. Most of the time dogs get diarrhea from an upset stomach and are fine in a day...but others it last a little longer, and can get worse if you don't take action.