Ditto what has already been said. We have always discussed CF in front of our girls, the good (if any!) the bad and the ugly!
My nephew also has CF and he has had a lot more problems than my girls have ever had and they hear us talk about things that he is going through. For example, right now Dalton is in the hospital b/c he was couging up blood...my girls have never had that happen to them but they understand it is b/c of his CF. They understand that he was much more sick even as a child than what they are.
And like others have said, I am very active in raising money for the CFF and my girls have heard me give more speeches about CF than what I can count! The last time was just in September. They hear me give facts...and then I get asked questions when I get in the car<img src="i/expressions/face-icon-small-smile.gif" border="0"> When they ask me questions about the facts I give, like life expectancy, I use that time to explain why it is so important to be compliant with all the treatments so that they can surpass those numbers.
I have always just taken the questions as they come. And when they are young, simple answers do the trick! As my girls have gotten older, the questions become more specific. For example, Kayla (10 years old) asked me recently how I found out she had CF. She wanted to know all about it. And how long we have been seeing her CF dr. Stuff like that.
Good luck with your little one! I am sure he will quiz you time and time again over the years<img src="i/expressions/face-icon-small-smile.gif" border="0">