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.