Thank you for your query.
Nose bleed can occur even without getting an injury
on the nose. I hope your son's blood pressure is normal. If nose bleed persists it is advisable to pinch the nose for 5 minutes by the watch so that bleeding stops. In case bleeding persists then he needs to be shown to a ENT surgeon who shall do nasal packing.
I hope I have responded to your query to your satisfaction.
Wishing your son an early recovery.