Sometimes nosebleeds are spontaneous and will resolve themselves without complication. True, it can be a pre-existing condition such as a cancerous growth in the nose or another problem, but being that it is allergy season and is very hot outside, and also that he is an older dog, nosebleeds are alot more common than you think.

Placing an ice pak on the dog's muzzle(not on the nose itself, but the nasal bone in front of the eyes) helps and that is all the vet would do anyway. It's always a good idea to get a radiograph of the dog's skull and nasal passages just to make sure there aren't any growths or abnormalities that may predispose him to nosebleeds.