What Causes Occasional Nosebleed?

My son used to have nosebleed when he was as little as 7 years old , somewhat like every 2 to 3 months.

As he grows older , the frequency of nosebleed reduced. Now he is already 17 years old and it happens once or twice a year since 13. Everytime it happens, I notice it's when the weather is cold or when he picks his nose hard during a running nose or when he is having a high fever. So, I want to ask Doctor for advise and please let me know whelther I should be alarmed. He refused to see a doctor as he thinks it's common for a number of his classmates do occasionally experience nosebleed as well.



Anxious parent