Like Haley said, diarrhea with no diet changescould be something to worry about. You're sure it's fecalsand not cecals? Because that would mean somethingdifferent. At the least, a fecal test would be inorder. It might be coccidia infection or otherparasites. It would also show bacterial infections.
A small amount of rolled oats can be helpful. Do not give himyogurt though because most rabbits are lactose intolerant and this mayupset his stomach. Your vet is probably just wanting it forit's probiotic properties. An actual probiotic such asBenebac (available through vets and in pet stores) would be muchbetter. Although the jury is out on how well those actuallywork with rabbits, it's worth a try. And Benebac definatelyhelps with cats that have antibiotic-related diarrhea- aka my kittens.
Also, I'd remove all treats, any fruits and sugary veggies like carrots, and maybe reduce pellets to make him eat more hay.