If everything in that department was ok at his foster home, I wouldn't worry just yet, either. Did you change his food from what he was eating? Make sure to get him lots of walks (exercise) and water. Maybe give him a few teaspoons of canned pumpkin (not pie filling) and if he's on a leash to do his business, put him on a long lead or rope to give him a little distance from the person. I've had dogs do this for a couple days but then things start to move again.