Elizabeth, yes, he should know- because eventually he WILL be told by someone else and it will cause confusion and hurt on his part and wonder why he wasn't told by the people he loves and trusts the most.
You may want to simply start by asking him 'have you ever heard us say 'CF' at the dr's office or when talking to other people, or the words 'cystic fibrosis'? Do you know what it means? Since he already knows he needs to keep from getting belly aches and to keep his lungs healthy, it will probably be easier than you think to explain it to him. Just telling him that the REASON he has to do those things (and no one else in the family or among his friends do) is because he has something called cystic fibrosis or 'CF', and he even has special drs that completely specialize in that to keep kids that have it as healthy as possible.
Odds are this is going to be a relief to him. Kids hear more than we think they do, and he's likely heard this term, picked up on your concern when he's gotten sick, etc and wondered about it. And it will certainly be easier for you just to have it out in the open. It's an important thing to just be open and as things come up, answer them. I agree it's not necessary to have a 'big talk' but just answering questions as they arise and explaining new treatments as they are introduced and why they are being done, is likely to be best.
In response to the thread in general...
In our case, we had to have 'the talk' initially because Emily wasn't dx'ed until she was 7.5yrs old (developmentally about 5 due to learning disabilities.) But it didn't even occur to me, frankly, NOT to tell her. She was going to hear us talking about it and she had to go to the doctor and we had to talk to the teachers etc... there was NO way we could hide it from her and yet still introduce the treatments. It was not too bad. In the 2.5yrs since then we have explained new things as they have arisen. It's been tough emotionally of late, since she has reached a greater understanding of the 'chronic' nature of her disease and has had a harder time coping. She was told by a friend at school she was going to die of her CF. The HARD issues are cropping up and those are harder to address than 'why do I have to do these nebs.' She's seeing the CF social worker right now to help...