My son was having tonnes of nosebleeds. We used to live in the west so it was very dry and much worse there. We took him to an ear, nose and throat specialists who told us that his blood vessels were enlarged and if his suggestions didn't work he would cortorize the blood vessels.
He gave us saline to spray three times a day and an antibiotic ointment that is actually used for eyes to put in his nose three times a day. Not sure if you have tried this that frequently. Something else I did was to make sure he was drinking more water.
All these worked for me and now we hardly have to do anything. I really think drinking more water made the final difference for us as his nose was constantly dry. You may want to see an ear, nose and throat specialists.