I'm sorry to hear your son has CF.
My advice would be to go ahead and cry. But know that this is not the end, but a new beginning. CF treatments have come a long way, you and your son will learn how to deal with this one step at a time.
Ironically, now that you know what is wrong with your son and begin to get treatments he will probably be healthier than before you knew he had CF. Marissa was almost 4 years old when she was diagnosed, and until she started doing her treatments she was always sick. Now she is able to run and play and keep up with all the other children and you can not look at her and tell she has CF. Honestly she is many times the only one who has not gotten a cold that went through the house. Now mind you that we do a lot of quaranteening here, but even so my fear when we first found out was that she would always be fighting for her life (NOT). That school would be impossibly dangerous (it's not), that we would have to keep her locked away to keep her safe (we don't). We use common sense, people who are sick don't come around her, and she knows to avoid the kids at school who are coughing, sneezing, runny nose, etc.
Marissa spent much more time in the hospital and was sick every day while she "only had asthma." Now that she has CF she can finally eat and absorb her food, and she is not sick that often. I wouldn't even say she is "sick" as often as my "healthy" kids are. The only difference is that when she does get sick we have to act on it faster than we do with the other kids.
You might actually find life easier as a parent who has had to take care of a child who was either misdiagnosed or diagnosed late. You now know what it is, and you will be shown how to manage it.
Good Luck to you!
Terry