I'm so glad you've joined us, welcome!
Poor little Merlin. Jennifer makes a good point to be sure you've ruled out other issues, and seeking the guidance of a Feline Specialist is definitely a smart thing to do.
You have a great idea trying an alternate protein food for your little boy in your quest to find the best food for him. Pinnacle carries a line of holistic, low allergen dry formula which I've known to be helpful. California Natural is often tolerated by cats with allergies due to its single, high quality protein source in each product.
Single protein foods are a good idea since the only effective way to manage food allergy is to avoid the offending allergens. First, though, you must determine what that allergen is, best accomplished by simply eliminating the suspected food from the diet and watching to see if the symptoms disappear. To complete the test, you can always check to see if the symptoms reappear when the food is introduced again. The majority of food allergens are proteins found in the diet. As you know, the first step is to change the cat food you are using. If your cat has been having reactions to a food based on beef, switch to a food made of chicken. If the problem is a chicken-based food, try lamb food, etc.
It must be so frustrating searching for the source of your little boyâ€™s sensitivity. I wonder if you would want to considering trying a home made low allergy diet? There are several recipes available online and even in books on the subject.