Do you know if any bloodwork was done?
My first thought is that he had something called hemorrhagic gastroenteritis (HGE). In this disease, there is pretty much just fresh blood passed (no diarrhea) and the dog can become really sick, get dehydrated, and in the worst case, die. HGE is thought to be caused by toxins produced by abnormal bacteria in the intestines--Clostridium, Salmonella, maybe E. coli. In some cases the toxins and/or bacteria can pass through the intestinal wall due to inflammation of the intestines (or perhaps there was some obstruction or perforation), then the bacteria can get into the bloodstream and make the dog septic. It sounds like he was treated appropriately, but the infection was too advanced.
Hope that helps--I'm sorry for your loss as I'm sure it was such a shock.