I am really sorry that your son is suffering from nose bleeds. Usually bleeding from nose in children occurs from front portion of nose. They occur due to dry weather, nose infections, nose picking. They resolve without any complications. First aid
to be done is pinching the nose for ten minutes which usually arrests bleeding, avoid aggravating bleeding by sneezing
, nose blowing, nose picking. But what really concerns me is the large amount and duration of bleeding and also seems to run in your family. Thus I would strongly recommend you get an ENT opinion, rule out local causes and bleeding disorders.
Be relaxed and take care.