Help with cleaning a dog's backside?

I'm dog sitting two Malteses, and one of them always gets poop stuck to his fur. I cleaned it the first time with soap and water, cut off some hair the second, but now he's done it a third time, and there's a lot. It's almsot like he didn't finish. He sits on the couches in the house, and I don't want him to get it all over the place. Is there anything else I can do?