could be from the dry air during the winter time, especially when heating is applied indoors, can use a humidifier in his room, make sure he drink plenty of fluid to stay hydrated, you can also apply small amount of vaseline just inside of his nares, on the nasal septum; also make sure he doesn't pick his nose(even that it could be exciting and amusing for a five years old, or sometimes for a 50 y/o), however, if his nosebleed continues and he has fevers, gum bleed, easy bruisiing, it's entirel