I just want to make one point, and then I'll leave it be, because people never like to listen to the whole "reason" and "logic" thing. He will find out what CF *really* means whether you tell him or not (and yes I mean that whole dying young thing --- it happens in most cases, whether you choose to believe it or not). If you'd rather have him find out on a website or in a book, then hey, that's your decision. But if I were you, I think I'd rather tell my kid myself.
Edited to add: That wasn't really aimed at the original poster, because you didn't say anything about wanting to sugarcoat or lie... that's just aimed at some of the responses.