What about a hydrolized diet? Z/d is the only food that is 100% hydrolized. It's a pre digested protein.
That might just work for him.
My Beau is a very sick dog with severe IBD and he is on pred and imuran to keep him from flaring. He did OK with the Royal Canin HP with higher levels of pred, but as soon as we dropped the pred, he started to flare as HP is not a hydrolized food.
In any case, I hope the HP works for Roger. It's suppossedly more palatable for them. If he starts to react from that, go to the Hills Science Diet, Z/D ULTRA. That's the one that's hydrolized.
I sympathize with you and for Roger.