Tonsillectomy - side effects severe urticaria

My brother in law had his tonsils out age 35 last November and was ill for weeks afterwards. now he has developed Urticaria and is on meds for it Celestamine and Zyrtec. He has got a long list of foods to avoid, and is now scared of eating as so many things he eats (which aren't on the list) also bring him up in hives. Clearly his immune system is on hyper alert. Can anyone suggest simple dietary changes or recommendations to help him? Thanks very much. Tweet