Your dog sounds seriously sick! I am surprised you have not rushed him to the Vet as of yet! Like with humans, whenever you see blood in the stool, IT IS SERIOUS!! Your dog may have accidentally ingested something like a nail, a piece of glass, etc which is tearing up his stomach. The longer you wait the greater the likelihood of your dog not surviving! Common man, this is common sense. Get your dog to a Vet NOW!!!