Too many things that it can be really, something in his stomach, intestines, an injury. I think you need to get him to the vet and make sure that he is going to be o.k. Hope this helps, good luck to you and the pup.
Why Is My Two Years Old Doberman Pinscher Pooping Blood?
Has your dog had any bones lately? My friends pit had the same problem but someone gave him bones that are hard to pass and he was fine in 2 days