Dog chronic diarrhea, this is an emergency?

I desperately need help. My dog is 8 months old and has chronic diarrhea. I have tried everything. We did a fecal twice- nothing. I had his blood tested- everything okay. He has been on different dog food brands (Canidae, Orijen, Acana, Before Grain, Wellness) and I have tried different types (chicken, lamb, fish). Still diarrhea. I now feed him just plain rice with canned pumpkin- STILL DIARRHEA. He has been on cortisone multiple times- they help temporarily but even then his stool is very soft, and by the end of the day he has watery stool. When I say my dog has diarrhea, I don't mean that soft poop, I mean literally nothing but water and mucus (sometimes with a tiny amount of blood in it) come out. My very last option as my vet said is to have an endoscopy done, which will run me $1200. Is there anything anyone can suggest to help me? My vet said even an endoscopy only has a small chance of finding out what's wrong. He can't have IBD or any other disease because my vet said the cortisone would have fixed it, at least temporarily. I have been to two vets and both have said they never have seen a case like this, where no medicine will respond. Please someone help me, I am losing hope and I feel like my dog is in extreme pain all the time. If he keeps up like this, he might die from malnutrition.