I totally know where you're coming from!
The guy I am dating now, well, we've been dating on and off for nearly 4 years, and I just now involved him because I was totally nervous. He always thought I just get sick a lot. But, I told him about my CF recently, not in really strong details, but the basics, and he cared so much he went and read two whole books on the subject, and even better, he's still around!
If you care about this guy, and you think he cares about you, I would tell him. Sooner or later it's going to come up, and if there is a chance that he will reject you for it (which he shouldn't) then its better to happen before you both get too involved.
I agree with Gem though, when the topic of my health first came up years ago, I too used the "It's like asthma," line. Eventually, he caught on that there was more to it, what with me always being sick, hearing me mention my regular doctor appointments, the fact that they were at a hospital and not a typical doctors office, and I never hide my therapy machines. little stuff like that leads to questions, which leads to more questions, and eventually an earnest interest or a total rejection. Now, after he read those books, I'm sure my bf knows more about the technical aspects of my CF than I do :)
Good luck :)