Your story reminded me of my own, with one of my German Shepherds. In the end, it was the chicken.
On reading your post I wanted to shout the word "chicken" very loudly, but I decided to read on and indeed, thankfully, it was Dottie that first considered the possibility of chicken. The chicken in the different food brands may well be sourced from totally different countries and the treatments given to that "bird" before and after hatching, may vary. This may be the reason why some chicken based foods are more agreeable than others.
The advice I was given by my vet to help Ronnie's problems was a simple diet of chicken breast and plain boiled rice. So, as a responsible owner, I did exactly this and what fired out the back of poor Ronnie, can only be described as luminous yellow mustard.
Cutting a very long story short, the chicken we eat from the supermarket shelves is in no way a cure for all ails and some thought is needed before anybody takes this advice from vets. I personally would obtain a sample of tripe based food, to see if things improve, before running up vet bills, for advice(like in my case) that may only make matters worse. In Ronnie's case, he ate chicken happily until I moved him onto a well known British, Chicken and Rice brand. From that moment on, Ronnie could not tolerate chicken. If I was to feed a breast of chicken to Ronnie with his evening meal, I will either get vomit within a couple of hours, or his entire meal on the kitchen floor, in a liquid form, by the following morning. Poor lad.
What happened with Ronnie propelled me into learning about dog nutrition and researching what is actually in some of these foods, the big brands with clever marketing. I was horrified. I have recently become a grain free only retailer and I will only sell products that I am happy to feed Ronnie and my other two. I do stock chicken recipe foods and I stand by their quality, but I will always ask a customer first and foremost about their pet's history of digestion and enquire into any history of food intolerances.
Let me just add, in the way of a disclaimer, there are plenty of good quality foods out there that contain rice and I would be happy to suggest a few, if asked. Personally though, I was glad to see that you have gone the way of a non grain food.
All the best,
Edit: Also Lee, make sure you do not feed him any treats with the rubbish in them. Don't let his day care feed him chicken or grain treats.
Stick with dried fish for treats, you can't go wrong.