Why does my son get frequent nose bleeds?

Very common: Children do pick their noses quite a bit and their small/sharp nails can traumatized the inside of the nose causing it to bleed. They even do this during sleep. Also, at times they rub their nose too hard too frequently. In dry weather, the inside of nose can be too dry and cracks, and rubbing can break the superficial vessels and thus bleeding. Consult your doc who can look up there... Good luck.

Other factors: Other factors can make nose bleeds more common. Allergies can both irritate the nose making it more vascular and cause a child to manipulate their nose due to itching, etc.. Some common treatments for allergies, like topical nasal steroids increase the risk of nose bleeds. Very rarely, nasal tumors found in children, like juvenile nasal angiofibroma can manifest as nose bleeds.