Puppy has loose stool... any suggestions?

I got a Australian Shepherd puppy two months ago, and from day one, he has had a really loose stool. Anywhere from diarrhea to a pudding-like consistency. I know it's gross, but I am wondering if anyone has some suggestions. The woman I got him from fed him Kirkland adult formula food, so I slowly switched him over to a puppy lamb and rice formula from Solid Gold. But it only slightly improved. So I talked to my vet, and she suggested something with brown rice, so I gradually changed his food again to a chicken and brown rice formula from Blue Buffalo, but nothing has really changed. I have taken in stool samples to the vet and they come back clean. Any ideas?