Here is the problem with hemoptysis:
Trying to seriously diagnose why it happens is a huge hit or miss. Some people cough up blood due to an infection, some do it just because they go into smokey bars and have no infection, some do it for little to zero previous reason. Some do it due to exertion (running out of the blue), while others do it just for laying down (has happened to me a few times).
Take your pick. The one universal aspect is, if you go from bleeding often, to not bleeding at all, that can mean that your lungs are so fried (bad), that they have lost the capacity to bleed (bad). I have had 2 bleeds this whole year, and last check I was at 99% lung capacity. I might be lower now % wise, I know my lungs have damage, but i'd like to think that i'm still capable of bleeding.
BTW, late teens and early 20's in a CF, are the years (if you are healthy) where you will probably bleed a ton, especially if you are social and drink, smoke joints, or go to bars/house parties with smoke etc. I bled like a madman growing up. Some people have BAD problems with hemoptysis while growing up, some level out, some never get past it...I hope he gets past it and chills out.
In general, the ultimate reason why we bleed is due to bacteria, and inflamation. If he can control some part of those aspects, there is a good chance his bleeds will lessen.