Glad to hear that Zack is still doing well. You will want to gradually add the Duck and Potato kibble--maybe 10 pieces of kibble the first couple of days and if that is okay then increase to 15 pieces for a couple of days etc. Dogs often do better with the chicken and rice itself rather than the chicken and rice kibble because kibble has lots of fiber and so is more challenging for the digestive system.
Karlin makes really good suggestions of other things to feed, and that works for most dogs, but for dogs with food sensitivities, like Zack, you really want to stabilize him on just a couple of ingredients and be able to wean him off the Tylan etc. You can add a Tablespoon of canned Natural Balance Duck and Potato to his kibble so he has a bit of variety. Once he has been on the Duck and Potato diet for a month or so and he is stable (no diarrhea/vomiting etc.), and is not on Tylan or any other drugs, then you can start to introduce other items if you want to "challenge" his system. However, with Trevor, we have stayed with Duck and Potato because that agrees with him and we had had such a hard time with so many other foods before we found something that worked! We had had 18 months of colitis, so we found that the Duck and Potato worked, we were thrilled to keep it simple
Roberta and Trevor (5 year old Blenheim)