I know what it is like to go through food issues and it can be really hard to find the right combination that will help your boy. The best place to start is to determine which foods he has done a little better on and which ones the worse. I use to keep logs of the ingredients and GA on how my girl reacted to everything and that is where I was finally able to find a link.
You said your boy has had diarrhea for 4 months–has he been eating the turkey, squash and oatmeal mixture during all that time? That mixture is high in soluble fiber, and the Royal Canin Ultamino and Gastro foods are also high in soluble fiber along with having prebiotics in them. If your boy is sensitive to soluble fiber, all these foods would make him worse. The Farmina Digestive is also very high in soluble fiber along with prebiotics so this food will probably give the same results as the Royal Canin food.
Has he been eating the THK Embark during the past 4 months and what were the results when he ate that food?
Have you tried any other carbs with the home cooked turkey?
Have you tried any limited ingredient kibbles in the past and if so, which ones?
Right now with the condition his gut is in, you are going to need to go very slow with trying to introduce any kibbles. You may want to try adding a teaspoon of plain kefir to each meal when you do another transition.
