Have you tried Go Sensitivity And Shine by Petcurean. Made for sensitive digestive systems and comes in a duck or salmon formula.
I have a dog with sensitive system and I find the simpler formula the better however she has never been diagnosed with IBS but does seem to have colitis if I change things in her diet too much. I haven't figured out exactly what triggers it but it can be hard with so many different ingredients.
I also find she does well on the honest kitchen force but it is pricey.