Awww...poor little fella.
I hope he is feeling better soon.
I dealt w/ something that sounds very similar a few months ago w/ Lexi. She started w/ just 'the runs' and then just as you described, started to have blood in it. She threw up a few times too. (I actually took a picture of it. Gross, I know, but I thought, "what if someone else asks and this *may* help them..." Go figure, I don't have the pic on my PC here...)
I'd changed her food and suspected that was it after the first day. Feed her chicken and rice, but it didn't clear up. I think it was day 3 that I took her to the vet, let them take a stool sample--gastrointestinal upset. In all probability due to the food change. As a pp stated can happen, she had some intestinal inflamation as a result and a higher-than-normal bacteria count (which is common when that occurs.)
She got some antibiotics and pectilin for her 'Hershey Squirts.'
She was back to her old self in around a day.
IMO, I doubt that after almost a week this is something that will just clear up without vet intervention/medication, especially when you have tried all normal 'quick fixes.' Just hang in there!