I have had this with my youngest daughter everytime the seasons change, especially as we get into the cold of winter. She has GUSHER nose bleeds for about 2-3 weeks and then once her body is adjusted, they go away. She usually gets them when her blood pressure goes up, like after playing or while fighting with her sisters (all joking aside.)
We moved here in September, and in the first month we were here, she had really bad ones. They went away about a month after we got here. The climate where we used to live was normally cold and damp and it was still in the high 90s here. I think these were the worst nose bleeds for her because we had just moved here, so she was under a lot of stress.
Try putting a humidifier in your son's room at night. If you are running the heat or air conditioning, and the temperature change between inside and outside is EXTREME then that could be causing it. Air conditioning and heat demunidify the air, so adding some moisture may help.
Nose bleeds do go away as kids get older and blood circulation lessens. If the number of occurences doesn't slow down or stop, you should probably see a doctor.