If he does not react to foods that are naturally high in riboflavin, then it may only be the synthetic version used to fortify certain foods that he is reacting to. And it may be what the riboflavin is derived from (i.e., the original source of the riboflavin) that he is reactive to, or could be a preservative used during processing. Do you know if he has any other food allergies? Also, may be coincidence...i.e., exposure to something else he's allergic to at the same time as when he eats the crackers, such as something in the packaging (a preservative, etc.). I do know that TBHQ is a preservative that some cracker companies use (Keebler, etc.) and I'm allergic to it (I break out in hives) so I know it is possible to be allergic to preservatives or other chemical additives in packaging or in the food itself. If I were you, I'd go to a Naturopathic Physician for help figuring out his allergies, because most allergy physicians don't have a clue when it comes to chemical allergies, and most Naturopaths tend to view the whole person instead of focusing in on just one body system or just one symptom, and thus they tend to be better able to diagnose obscure or unusual health issues more easily. Also, they tend to be more familiar with chemical sensitivities, and may have ways to treat the allergy so he can eat the food without a reaction.
Gluten & Sulfite Sensitivity, Many Food/Inhalant/Medication Allergies & Intolerances, Asthma, Gut issues (dysmotility, non-specific inflammation), UCTD (Lupus?), Osteoporosis, Anemia, T- & B-Cell Lymphopenia, malabsorption/malnutrition, Lyme Disease (Igenex Lab IgM WesternBlot positive/CDC negative), etc.
Meds: Pulmicort, Injectable Vitamin B12, Herbs, Nutritional Supplements, Homeopathy.