he should be taking an anti histamine at home for 5-7 days. for the rebound/relapse that occurs after 48 hours. (no need for steroids)
most of the time the cause of the urticaria is not known. it coukd be a new medication or even an old mediction he has been taking. or a food or spices that he developed an allergy too.
he should be seen by an allergy specialist if his symptoms didnt improve.