Many horses develop small bumps or raised areas on the skin called hives, or the technical term Urticria. These are caused by an allergic or hypersensitive reaction. The cause can be topical from an insect bite, fly spray, shampoo, a saddle pad with detergent on it, or systemic from a food or drug that may have caused a reaction. Generally hives are not life threatening, with the exception being if the throat area closes and the horse cannot breathe.
If they do not resolve themselves in a few hours, it is necessary to call the vet who can administer drugs to reduce the swelling and discomfort. Try to identify what could have caused the hives by checking food sources and external causes in his environment.