have you checked www.wholedogjournal.com? They have great articles every year with their choices for commercial food which is US made and ingredient sourced and complies with AAFCO standards and they list waste product %ages like ash and gluten.
You might have to go to back issues and pay for searches but I'd be glad to dig up my 2009 articles and look through. I might try canned non fortified food since vitamin supplements are often the culprits behind darker urine-hence darker saliva and teeth staining. Drinking more water would also help if you can add some to food as "gravy" or add doggie flavored electrolyte water to drinking water to encourage water intakes.
Also a vet check is prob in order since some teeth staining is a sign of things like kidney disease and improper filtering of waste from the body. Do you add probiotics to your dog's food. Just a thought along with a change to a better quality more nutrient bio-available food. If he's pooping most of it out he may not be absorbing the nutrients well either because of quality or a health issue like Addison's disease, celiac disorder, etc.
Try distilled water for a while too. 8 weeks is usually the time it takes to see if a dog is sensitive to any specific food ingredients. Have you been to a canine dentist too?? They might be able to give a more specific reason since they are so specialized.