Fletcher has (relatively) mild IBD. He was never scoped, my vet just said, let's treat it as IBD and see what happens. He started out with pred, Flagyl, and something else I've forgotten, to get get his bowels stable and to get rid of any parasites. He's a poop eater, and until I caught on to this, he ate feral cat poop as well as his own He also went on a limited ingredient food, venison and sweet potato, I think (this was all over three years ago). Through trial and error, I've discovered his food triggers (many), what he can eat in small amounts and what he can't eat at all. Now his IBD is controlled through diet, with Flagyl for an occasional flare up. Luckily, Flagyl works well for him. He is now raw fed, and that has been a life saver for him.
I think, unfortunately, each dog with this is a little different, and you have to find what foods work and what doesn't. Grains and starch seem to be pretty common culprits, although they don't bother Fletcher. He can eat any protein I've tried him on, but absolutely no veggies, and only a few fruits.
What should Beau eat? He seems to do fine on beef and chicken, yes? Maybe he needs to stay away from grains since he has been doing well on BG. My advice, FWIW, keep the ingredients limited and stay with them unless he gets worse or quits eating. It takes time, especially since he's been through so much. He's so sensitive, maybe just having Polli through with chemo will help, who knows?