It sounds like he has small bowel diarrhea.
Here are the differential diagnosis:
PRIMARY SMALL BOWEL DISEASE
1. Inflammatory bowel disease (ex. lymphoplasmacytic enteritis, eosinophilic enteritis, and granulomatous enteritis) It is very responsive to metronidazole.
www.VeterinaryPartner.com/Content.plx?P=A&A=598 More on IBD
3. Infection (ex. histoplasmosis, salmonella, clostridium perfringens, and phythosis)
Have a sample of the feces sent off for culture and sensitvity.
4. Partial obstruction (ex. foreign body, intussusception, or a mass)
5. Small intestine bacterial overgrowth
6. Gastroduodenal ulcers
7. parasites (ex giardia, coccidia, worms)
1. Exocrine pancreatic insufficiency (ex juvenille pancreatic atrophy and chronic pancreatitis)
www.VeterinaryPartner.com/Content.plx?P=A&A=1627 more on EPI
2. Hepatobiliary disease
1. Dietary intolerance or food allergy
1. Hypoadrenocorticism www.VeterinaryPartner.com/Content.plx?P=A&A=608
3. Drug administration
If you need any of these conditions explained further, I would be happy to. It sounds like you are a MD yourself or in the medical field and I did not want to insult you.