If the stool is still red he may be trying to pass a sharp piece of something like plastic, this could cause sepsis and kill him slowly by leaking gastric juices into his abdominal cavity. If the stool is dark brown then the diagnosis isn't any better, that kind of blood would have been pooling in the bowels for a while which is caused by a legion that keeps bleeding and it not healing. If you can afford to take your dog to the vet, do it.
