Suggest Remedies For Recurrent Hives In A Child

My son is 7 years old..and he is asthmatic..about 3 weeks ago he was coverd with hives...we went to a dermatologit and was gives medicine asked to stop allergy foods ..,and keep a record about his diet.My problem is he is getting the rash again and again..he only takes vegies,rice,bread,jam,pears,apples...not even milk because of this..can you pls tell me the reason behind this?