Hi,
I think it's important to establish yourself at your job, show them what you can do and that you're a hard worker...and then let them know about CF after that. I have also found that it's better for my bosses to know so that they know why I need days off from work for appointments, tests, etc. I don't want them thinking I'm just taking time off for no good reason.
My current boss I told after about 6 months of working at my job. We were at a work conference and we were actually sharing a hotel room when a similar subject came up and I just told her. She was like, "Oh, that's why you have that little cough." Hehe. So now there's no more questions or any problems with needing days off for medical things. And come to find out, her cousin has CF too (she's in her late 40's and has two kids)! But I think because I work hard at my job and I do everything to the best of my ability, she is willing to work around my crazy schedule and accommodate me in any way.
Because your boyfriend sort of works in a more private job (where everyone pretty much knows each other), I think it's important for him to be open about it. But if he can, hopefully he can prove that he's a hard worker, reliable, and dedicated to his job before telling his boss. If it happens that it must come up sooner (he needs IV's, etc) then there may be no other alternative than to tell his boss.
It really all depends on the person and how comfortable they are. I have found that it's much easier to be honest though than have to sneak around with lies.