Suggest Treatment For Weakness And Nausea When Diagnosed With Food Allergy

Hi, my 11 year old son has had food and seasonal allergies since he was 2 years old. He has grown out of many of them, but currently is allergic to peanuts, tree nuts and corn. (Along with pollens and dust.) We are vigilant about the peanuts/tree nuts. He has had a weaker reaction to corn, but we still try to avoid it. At times we allow him to have things with small amounts of cornstarch, dextrose, maltodextrin. I m worried about his overall immune system. Over the summer he had 2 viruses according to his pediatricians. One he had several days of 104 degree temps. Both took a long time to get over. He has energy most of the time and plays basketball for long periods of time. However, he often feels weak and nauseous frequently throughout the day. We have started a food journal. Any additional suggestions?