I'd trust a positive skin test, before I'd trust a negative one.
Probably not what you wanted to hear. *I* would not
feed my child corn and soy after a positive test. Reactions could be internal and not obvious yet. However, each subsequent reaction is generally more severe with continued exposures, since more antibodies are developed to the protein in the corn and soy (egg).
In order to not cloud the issue, I'd not try eggs internally. I might consider a topical egg skin application and see if the skin turned red. That would convince me not to feed my child eggs. Some people only react to eggs from chickens who have been fed corn, or fed soy grain. Farm raised eggs, where the chickens are healthier and able to forage and eat bugs and plants could be less allergenic. But, personally, I'd wait until my child was about 5 or 6 when he could tell me that his throat was scratch or itchy before testing something which was SPT +, even in the past. And I'd have an EpiPen in hand.
I'm a bit paranoid about IgE allergies.
Does he have any bowel, behavior, sleep, or eczema issues currently?