If you want some one-on-one help with both using s boulardii and food to try and deal with this issue join this FB group. We're pro-raw, of course, but also full of really smart members that really know their stuff about IBD and diarrhea and have helped hundreds of cats. They have a good protocol in place to help cats when vets just keep throwing meds at you.
https://www.facebook.com/groups/RawFedIBD/
https://www.facebook.com/groups/RawFedIBD/