Chappie is a good universal food which many dogs will tolerate when they don't eat much else. But talk to your vet and ask him to consider prescribing Budesonide. It is a steroid, yes, but one which 'targets' the right areas. It does NOT lead to weight gain.
I had a dog with no symptoms for 5 years, then the kibble we always fed was discontinued and so we looked for something as close as possible to the same composition to mix with Pedigree Chum, veggies and self imposed diets of paper tissues. Within a couple of months, she showed all the symptoms of IBS, including eating huge quantities of grass to throw up. In other words, there was an 'itch' in her tummy she was trying to get rid of.
Tests revealed that the whole of her stomach lining was raw and red.
We put her on to grain-free at the vet's suggestion and he started her on a course of budesonide. One pill a day. She very obviously benefited and the dose was reduced to every other day. I kept her on 3 pills a week, except if I saw ANY signs recurring, in which case I put her back on daily medication for a couple of weeks. The vets know me and entrusted me with a supply of the steroid.
It was easier to put the entire pack on grain free kibble - it got too difficult to feed one dog apart from the rest of the pack (of 8).
The wee girl lived another 7 years on the Budesonide ! And the rest of the pack came to no harm on grain free kibble.
Its worth a try, so talk to your vet.