The only thing that I can suggest is that you change the diet that he's on. I would switch him over to a bland diet of only boiled chicken and white rice. Feed ONLY this for the next 7-10 days. If he improves, feed it until the symptoms are gone. When the symptoms resolve, start to add in a high quality dog food little by little until you phase out the chicken/rice.
I would also collect a fresh stool sample and take it to the vet's office. Most vets won't charge you for an office visit if you bring a sample in (and not the dog) and get a fecal float. Then you can find out if parasites are the root cause. This is the most common scenario.
I hope this helps!
Let me know if you want to discuss this more.
