Has the vet run a stool sample check?? Some parasites you can see this happen when they are in the shedding cycle. Also, if there is also bouts of diarrhea I would definately check for giardia (a protozoan parasite) which often has to be requested as it isn't always a in house lab check.
If parasites are ruled out, then I would go for the endoscopy.
I have never run into this in any of my animals, however I am in agreement with you as far as your vet giving you the runaround and I am in agreement with your second opinion vet that an endoscopy is definitely in order at this point. A tear of polyp is a distinct possibility, as is an internal hemorrhoid that cannot be visualized by simply examining the dog.
Is the blood bright red or is it dark? If it's bright red, then the source is close to the anal opening because if the blood was coming from further in, it would be a darker red and if it was coming from the stomach or WAY further up the digestive tract it would be black and tarry looking.
I can definitely sympathize with you on what you are going through, though, even though I have never dealt with this type of thing. Just having a pet with an ailment that cannot be diagnosed easily is very upsetting and nerve-wracking. If only they could talk and tell us how they feel! Please keep us updated on how things are progressing with his case. I am sorry that I cannot be of more help to you.
Ghilly