Sorry to hear Meg as I am dealing with the same thing and have since we brought the babies home. They were negative for worms so the vet did cytology (direct smear under microscope and sometimes a special light) for bacteria and that is what they have - plus it keeps changing as one clears up. Hope you get poor Gabe cleared up and that he does not "share" with Gracie.
I will also try the chicken baby food for blandness. I am ready to tear my hair out. I forgot to ask about "amphorol" - doc wants to try Flagyl next but I will ask when I call.