look around your house for little orangish dried rice looking things. that's dead tapeworm segments which can cause the straining and the loose stools. wet food really isn't recommended for dogs. until you're certain what is causing it, i'd keep pooch off it. does your vet have a daytime on call person? at least call them and leave a message requesting a call back. very good advice on the rice and hydration. i am not sure about the pepto though, i'd wait and ask about that on the call back. don't let him run around too much, if he's like my dog that just gives them the urge to crap more.
luck!