Did the vet check his stool? Many things can cause diarrhea. Parasites Giardia, coccidia, samonella, E-coli, and an overgrowth of bad bacteria in the gut, stress, food allergies, viruses like Parvo and distemper, etc. Mucous in the stool could be from colitis or Giardia. Probiotics help to restore good bacteria in the gut, necessary for digestion... I give it to mine with every meal. Letting a pup/dog go on too long with diarrhea can have adverse effects on their health, dehydration being one that can be deadly. Pedialyte to drink would help too. After this amount of time, IMHO, I would have a stool check and bloodwork done asap.
ASPCA | Diarrhea How To Treat Diarrhea In Dogs | Acute & Chronic Diarrhea Remedy Diarrhea & Dogs - How to treat Dog Diarrhea