Get Your Dog Care Questions Answered by Experts
Has there been any change in his diet?
Any bones, rawhides or non food items lately?
Any vomiting, or diarrhea?
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.
I suspect that he may have ingested something that is causing irritation but it could be IBS as well. Your going to need to take him in for a fecal smear and a examination to make sure there are no underlying issues causing the problem.
If it is going to be a few hours or longer before the vet can get you in then we need to try and settle his stomach down until the appointment. I would recommend starting him on a bland diet of boiled chicken and rice to help settle his stomach down (1 part skinless chicken Breast to 3 parts white rice). I am concerned about him becoming dehydrated from the diarrhea so give him unflavored pedilayte to replace the electrolytes he is losing.