I echo the pumpkin. Both cats like it and it seems to clear up loose stools in a day or so. It also freezes well so I buy a big can and decant it into a few jars in the fridge and freezer. Smudge's Mom Gina posted once that they freeze well in perfect portions in an ice cube tray. You do have to stir it when it's thawed to get the consistency back.
As for any of the hypoallergenic foods out there my two REALLY disliked them, (think hunger strike), and I've tried them all over the 8 years after Tiki's IBD diagnosis. Several of them he was sensitive to as well. Seems that his body can recognize any meat with feathers no matter how small they make it molecularly. This is what they've been on for many years and both like:
https://www.hillspet.ca/en-ca/cat-food/p...ormula-dry
It's not cheap but they do clean the factory between it and runs of other foods, hence the higher cost. Their coats are really lovely on it.
Sorry to hear of Shadow's troubles. He sure does like to keep you on your toes doesn't he?