Unfortunatly, there is nothinng you can do unless you ARE a doctor and can check him out yourself.
Yes, constant headaches and nosebleeds are potentially the sign of something serious. But you can't force a friend to go to the doctor. The best advice I could give you is to say something to the tun of, "LOok. I love you and I care for you and I'm terribly worried about you and can't be as dismissive of your health as you are. So, if you can't go to the doctor for your own good, please just have a visit for me."
And if he won't, he won't. It's terrible if he won't and this has been going on and continues, but you are limited in what you can do (save maybe hinting to his parents as well that he has been reporting these symptoms, or dragging him tino the school nurse forcibly).
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