This could mean that your dog could have a parasite infestation. Parasites such as hookworms. You should check your dogs gums for discoloration (lightpink or whiteness.) If there are any signs of discolorations,you should deffinately take your dog to a vet.
It could be that he has a hemmoroid or the worst scenario blood in stool could be a type of diverticuli that is commonly found in older people.