Have you checked out allergies? My baby (who is now 34) has been having nose bleeds the majority of her life. She would usually wake up with blood on her pillow. I took her to the doctor and they could give me no reasons execpt maybe I needed to check her pillow for dust mites, feathers or whatever. I would be interested in knowing how you treat them. What is his diet? Green vegetables with vitamine A like spinach, brocolli, kale, and the likes allows the blood to clot easily. This might help. Plus you can always talk to a pediatrician about what condition his over all health is in and that might give you an idea of what he may need to stop the nose bleeds.
My baby still has them but not as much as when she was a baby. She has children of her own who don't have nose bleeds so her's is not hereditary either (I rarely if ever have any). Good luck, when you get this all taken care of drop another line so you can pass it on the rest of the folks. R.