Something to consider - this could be a reaction to something on the wrapper. Most people end up sticking the outside of the wrapper into their mouths. Pixy Stix are often repackaged in facilities that repackage nutty candies and could've gotten contaminated that way, and by whoever or whatever else touched the wrapper before giving it to him. The only other thing I'm thinking is, the flavor has 'Maui' in it. Maui to me says tropical fruits, like kiwi or coconut, which could be a new (or just up until this point unknown) allergy for him.