Hi and welcome to the forum! I am glad that I can help you.
You want to know the cause of the frequent nose bleeds.
Individuals may experience frequent nose bleeds due to injuries of the nasal blood vessels, weakening of their wall due to lack of vitamin C, or other issues connected with the coagulative system. If the nose bleeds are severe and need more time or treatment to stop, there might be an underlying medical condition connected with the coagulative system such as decreased number of platelets, lack of vitamin K, etc.
According to the data that you described, your son experiences nose bleeds after getting a knock near the nose. He might need to get his coagulative factors checked to exclude any medical condition connected with the coagulative system. He may consider taking vitamin C or bioflavonoids, in order to strengthen the wall of the blood vessels.
Please keep in mind that I provide medical information only. I am not able to diagnose medical conditions online. Please contact his doctor for further advice and information about diagnosis and treatment options.