Hello and welcome to HCM SInce your son has urticaria, he is generally prone to allergic reactions. Has he been taking xyzal as a regular course everyday or only before going to swim? It is possible that his allergy levels in the blood may have increased due to reasons such as repeated exposure to the cold, or certain foods, dust, insects etc, any infections,worms (consider deworming). This could have worked over the effect of the xyzal. Since you have not mentioned age, puberty, hormonal changes esp thyroid can increase chances of hives. You can check his blood levels for eosinophils and serum IgE. In case of other causes a skin prick test can help. I would advise you to take a regular course of levocet+monteleukast tab every night for 2 weeks and then taper it off and check. while on treatment, avoid all possible allergens. This will stabilise the allergy levels. Apply calamine over the rashes. If these hives are persistent even on treatment, do consult a dermatologist and check required investigations as advised. He may have other allergies. Hope this helps.