If you tell us what brand your dog is currently eating, we might be able to help you spot the ingredient(s) most likely to be an allergy suspect.
Some food items considered to be possible allergens for some dogs,
cannola oil and other GMOs,
but all dogs are individuals, and your unique dog could have his very own allergy.
Maybe best way (besides have vet do some allergy testing on dog)
would be put dog on a dog food with few ingredients. You could make it yourself, taking care to add calcium ingredients (like so much teaspoon ground eggshells per pound of meat, etc, if you don't give raw bones, etc)
or follow a recipe for very plain, few-ingredient, homemade dog food recipe from a reliable site. It's a bit more complicated (not much, but a bit) than just throwing meat to the dog, you'll have to google some recipes. Not all homemade dog food recipes are balanced or healthy, sorry, i don't have a link to offer for that one...
You could either try one of the kibbles considered "hypoallergenic"
or "low ingredients"
One sort of has to be a bit careful about some "hypoallergenic" dog foods, as some of them run low on meat or protein.
and see if dog is still itchy/scratchy/skin problems/chewing his feet or legs, etc or whatever.
If you have the fund$ to do so
, another thing to consider would be to offer your dog raw food,
or one of the raw pre-made
OR get one of the mixes that one just adds their own meat to it,
there are others, as well.
(be aware that raw feeding you provide on your own, using meat, raw bones, and organs, does require some
education to achieve a balanced diet, http://www.rawlearning.com/
Then, if symptoms disappear, and you are happy with and can afford current diet,
you could stick with it.
If not, you could watch ingredient lists closely and slowly try to swap to a different food, if dog gets itchy again, study the list of the food.
Dogs can also develop allergies OVERTIME. My last dog, after eating eggs for years, no problem, decided he could no longer do eggs. at all. ever. Well, he vomitted for all eggs after he got older, which is not usually considered an allergy, so much as an intolerance.
but dogs can and do develop allergies to foods they previously did fine with, which is good to know.