Novel protein would be a protein that is more rare. Something he is unlikely to have been exposed to. An example would be Zignature’s Kangaroo diet. LID means limited ingredient, just the essentials so it helps narrow down where the intolerances are coming from. Allergy tests don’t really tell too much, they give false negatives and false positives. The best way to figure out intolerances is trial and error. Since you know he can handle fish, i’d find a fish LID and keep very close tabs on ingredient panels, keep a list of what things aren’t working. Natural Balance, Nature’s Variety, Zignature, Acana, Fromm all have diets that may work. Buy small bags so you aren’t out all the cash, but it will take some time and money to get it pinned down. I don’t love Natural Balance, but it can help with this type of issue as they have a lot of options in the limited line. I’d use some digestive aids in the process as well- The Honest Kitchen’s Perfect Form is a good one.
I am in agreement that dogs are technically omnivorous, but I also find it more in the scavenging nature of canines, as they do what they need to to survive, but definitely have a carnivorous bias. Dogs need meat. That’s how they thrive. Don’t cut out all options of animal protein until you’ve exhausted them.
