CF is one of those things that can very easily slip up on you in my experience.
As a child I was pretty much the normal guy, in fact I was very athletic and it was near impossible to even tell I had CF unless you did a sweat test (I was very good at hiding my emzymes).
However, as I and many others learned (and were told numerous times) symptoms can change rapidly and drastically.
This isn't so much meant to scare you, but rather to warn him. It is easy to ignore this disease when your feeling well, but when your sick it is a real struggle to try to get back what you lost.
Do your best to convince him, but try not to force him. He might feel disconnected from the majority of us in the adults section right now, so I would advice him to check up with some of the marathon runners in the exersize section of this site and have him ask why they feel it is important to keep up with this disease.