How can I help my child's food allergies, without know exactly what he is allergic to?

Consult an allergist. An allergist can help you by carefully evaluating your child's history of potential reactions to foods, and selecting the appropriate testing to confirm the clinical impression of allergy. It is also possible that your child does not have any food allergies at all.
Allergies. Pediatrician can order skin patch testing to find out allergens & recommend what to avoid or eliminate from diet.