Cold urticaria is just another way of describing the simple hives that some children (and adults) develop on areas that are exposed to cold. They can be triggered by chilly weather (especially wind), swimming in cold water, or in rare cases even drinking an icy beverage. They can look scary, but are usually nothing to worry about -- and the rash usually goes away on its own as the skin warms up.
Most of the time cold urticaria doesn't require any treatment other than getting out of the cold environment. However, in rare cases, some kids can have extreme reactions beyond hives, including swelling of the lips, tongue, or throat, which can usually be cleared up with an antihistamine like Benadryl. It is rare to have full anaphylaxis (a severe, full-body allergic reaction) with cold exposure, but you will need to be more careful about exposing your child to the cold. Bundle him up well when the weather gets chilly, covering as much exposed skin as possible, and instruct him to never swim alone as he becomes older. This way, someone will always be there if he has a severe reaction and needs help getting out of the water. Your doctor may also suggest giving your child a dose of non-drowsy Claritin, an antihistamine that can help stave off symptoms, about 30 minutes before cold-weather activities like skiing or swimming in chilly water.