The shots can take months to take effect and a lot of times those allergy tests are wishy-washy, especially with food items. That being said, to be cautious I'd avoid corn anyway if that popped up on the list.
What were his clinical signs?
In order to pick an appropriate food IF he has food allergies, you need to evaluate all of the foods he's had in the past. Often times chicken and beef are the top food allergens. The protein source in food tends to be the culprit, so your goal would be to pick a food with a novel protein source. Things such as Venison, duck, salmon, bison etc tend to work. If the allergies are bad enough, the hydrolyzed Rx food may be the only way to go.
I'd look for foods that are "Duck and green pea" or "venison and sweet potato"... things like that. I just switched my aunt's dog's food because of chronic ear infections and told her that our last dog had food allergies and was on Nature's Recipe Venison and Rice which worked very well for her: Nature's Recipe Venison Meal & Rice Dog Food - Food Center - Dog - PetSmart
That may be a good place to start. Wellness also has some good choices like this Duck formula: Wellness® Simple Duck & Oatmeal Formula
You'd have to feed him this food and ONLY this food for a minimum of 4-6 weeks. No treats, no table scraps no nothing. Otherwise trying this is pointless.