Well, I think there's a difference between tearing it up and playing with it, and eating it.
If they are eating it, they need more fiber in their diet. OR he could be hungry. Try adding in some treats during the day. In the meantime, close the bathroom door or put the paper up on the counter
That could get expensive, and quite embarrassing for needy times!
Dogs tend to treat themselves, just like when they eat grass they are trying to either settle their stomach or make themselve vomit up something that isn't sitting well.