I'd call your vet again and ask if he thinks METRONIDAZOLE is something that would do your kid a world of good. It sounds to me like the natural bacteria levels in his digestive tract are just out of wack and he's having a hard time recovering.
Yogurt is always a good additive in cases like this since dogs really can't produce lactose it should bind him up a little while also contributing good bacteria to his intestines.
Otherwise Metro. has ALWAYS cleared up my kids w/ just one dose, two at most.
The key thing at this point is to make sure he's not dehydrated! If he won't drink water, you can try giving him Unflavored Pedialite or maybe make him up some broth w/ bullion (sp?) cubes.
Good luck.
Doesn't sound like anything too major though. If it gets worse, then of course take him in!