Get Your Dog Care Questions Answered by Experts
Blood in the stool is typically one of two things a parasite infection or a blockage of some type it is often referred to as Colitis. Though there are other causes such as IBS (Irritable bowel syndrome) and other more serious problems such as Cancer.
If the dog is not vaccinated or has only recently had his vaccinations then this could be Parvo which is a very deadly illness if left untreated. I would strongly suggest getting him into see the vet today or your nearest emergency vet.