I'm with Jax08 on continuing to feed RX food until you and your vet have a plan, if it's helping your dog. A dog with true food allergies that are uncontrolled is going to live in a state of inflammation when it's eating food that sets it off. That has a cascading effect on the entire body. These dogs can end up with entire GI tract inflamed all the way through to to the anus. It's very painful. My dog stopped eating food that made him sick and would go many days without eating -- he'll easily lose 10% or more of his body weight by starving himself before he'll consider eating food that makes him hurt. So while the Internet Commentariat has lots to say about not indulging picky dogs, most of them haven't owned true allergy dogs where it's not being picky--it's survival instinct because of internal pain. People who haven't lived this don't realize how bad it really can be for the dog -- if the RX food turns off the "fire" of the inflammation and suffering, and it gives you breathing room to figure out next steps without him suffering, feed the RX food! I was lucky to figure out we had an alternative, but until you know that, you're wise to just keep the dog stable.
Here are a few thoughts on kibble:
1. There's some published research out there in a vet journal that I've read in the past that found that all of the non-RX limited-ingredient foods the researchers tested were contaminated with ingredients not on the label. The RX HP foods tested were not similarly contaminated. My recollection is that they learned that in between runs of food, the non-RX/L-I manufacturers don't scour the equipment so whatever leftover bits were in the machine from the previous run may make it into the next one too. If you have a true food allergy with a high level of sensitivity, you may not get the relief you need without an RX food, if you're going to feed kibble. OTOH, for some dogs it may not be in high enough quantity to trigger them. It's a bit of a crap shoot and possibly doesn't isn't a good indicator of whether the dog is allergic to that protein, due to the likely contamination.
2. Among non-RX foods, the rescue I work with has had the best luck with Wellness Simple. The Diamond Care hydrolyzed protein linked above was okay for some too. However, some need RX diets. Given the contamination issue, you might get lucky on a L-I food, but you have to be open to the chance that you may not find a limited ingredient diet that works. However, do read up on our DCM mega thread here -- some limited ingredient foods with "exotic" proteins are supposedly correlating to illness in some dogs, esp. Goldens, but nobody is sure why or even if it's really the food causing it. Be fully informed of the controversy before choosing these foods and follow the developments of the on-going research if you choose to feed it....and *maybe* throw in a taurine capsule just for good measure. Also, some L-I foods are too low in fat to support optimum growth, so look into that too in a growing dog.
3. My vet's instructions are that a true elimination diet takes 6-8 weeks PER food tested. So you need to stay on the RX diet by itself that long. Then you can add in one ingredient at a time, again, giving it enough time. If you vary the diet during this time, you re-start the clock. That includes treats, by the way. Once you find a diet that he can handle and identify allergens, after about 6-12 months of being stable on the good diet, then the protocol is to "challenge" them with the suspected allergen to confirm it triggers a response.
4. Some research is suggesting there's a gut flora component of food allergies in people. That may be true of dogs too, so think about whether he needs to be on a probiotic to try to help over the long term. Many ingredients in kibble are Round-Up crops, and Round-Up has been shown to harm gut bacteria in people...again, it may be true of dogs too. My pure speculation is that some of these allergy-dog bodies may be reacting to the ultra-processed, industrial sourcing of the offending ingredients in kibble -- but there's no research on that AFAIK
. With a a high-quality, balanced raw or home cooked diet you can focus on "clean" sourcing of ingredients and minimal processing, which *might* help some dogs. Otherwise, don't be surprised if the RX food may be all he can handle because it's processed to not trigger a reaction.
5. In addition to the base mix I linked above, you can find other minimally processed base-mix options from Sojo's (with or without oats), Dr. Harvey's, Volhard Nutrition, and a similar add-in product Balance It. Alternatively, once you know what ingredients work, you can consider whether it might be worthwhile to pay Balance It (a UC Davis-affiliated, vet nutritionist-run service) or someone like Monica Segal to formulate a balanced recipe for home meal prep.