Some Holistic vets recommend you take them off everything for a short period and feed them a "novel" protein which is generally game meat (something that is not commonly fed or found in most commerical foods) and then after awhile slowly introduce a different protein one at a time to try and pinpoint the culprit.
Yup, that is a standard elimination trial

The reason for the novel protein is that it is impossible to be allergic to something you have never previously been exposed to. OK, in a few extreme cases, you could react as early as the second exposure, but that's actually quite rare - allergies are far more normally something that build up over time and after repeated exposure.
The point of the "novel" food, thus, is that it is
unique for the dog being fed it. It does
not matter one iota whether the food is commonly fed to others, is game, etc. The singular critereon is that it is something that YOUR allergy-suspect dog has not been fed previously, and therefore cannot possibly be already allergic to.
If you choose to do an elimination trial (which, I must say, is far easier for those feeding homecooked or raw than for kibble feeders - the latter of whom cannot possibly deal with ingredients truely singularly), then you can pick any meat you like to start off on. As above, the only critereon is that Cyrus has never previously been fed it. So you might have to look around for a decent source - as to do an elimination trial properly, he should consume
only his "unique" meat for 12 weeks (yes, 12 weeks - that is how long it can take for allergy symptoms to subside).
Of course, symptoms may subside sooner

But you need 12 weeks for certainty. IF, in this time, the symptoms do subside with no other changes to his general environment - then you have proven that a food allergy exists. That is information worth having: and you can then embark on the process of adding foods back into his diet - one by one, and with space in between - to determine which ones cause a reaction and which don't.
IF, however, you get little or no improvement over your 12 week trial - then you have actually proven that his problem is NOT a food allergy. Which is also information worth having!
Last possibility is partial improvement, which probably means that yes, he has a food allergy or sensitivity (and you continue with the elimination trial as above to find out what is problemativ and what isn't), but that it isn't his only problem - and you're probably dealing with an environmental allergy as well. It is perfectly possible to be allergic to several things, after all, and food allergies only make up about 10% of them.