Oh the poor little guy! And poor you — it's so frustrating having a sick puppy and a vet who can't tell you what the problem is.
Have you tried doing some research on google? When ruthie got sick (liver issues) the first thing I did was start doing research so that I would have a better idea of what was really going on - and I had LOTS of questions for the vet after. Have they ruled out inflammatory bowel disease?
Here's a few links I found — not sure if they'll be helpful, but it's worth a try? Maybe you'll find something in here that the vet hasn't thought of.
Diarrhea in Cats and Dogs - Pet Health Information from Vet4Petz.com Ask the Veterinarian: Liquid stool and Vomiting, inflammatory bowel disease, dry dog food Malabsorption in Dogs Chronic Diarrhea in Dogs
Welcome to YT — sorry it's under these circumstances — I hope your little one feels better soon!