Allergies

My son has really sensitive skin, at first he had bad exzcema that flared up often and i had that usnder control after about 7 months but now he has an allergic reaction it seems to food and its been 3 weeks, i feel that everything he eats is giving him a reaction! Do you guys have any suggestions on what to do or creams or whatnot. Hes always just sitting there scratching or tossing and turning because he is itchy.