I'm a breastfeeding mama with a 10 month old baby, but in our situation I am the one with CF. However, I've talked with my mom extensively about the subject (breastfeeding) and she said that my sisters and I (all with CF) ate A LOT more frequently than my brothers did (no CF). She mentioned how it seemed like ALL she did was sit on the couch and let us nurse... ALL DAY! To this day, I feel best when I eat smaller amounts more frequently throughout the day.
As far as being gassy and fussy... I can tell you that the stomach and digestion issues related to CF are no fun! I imagine your little guy may be in a lot of discomfort. But, as others have said, breastfeeding is the one of the best things you can do for your baby. If you are having issues with milk supply, allowing him to nurse is the best way to help with that.
I am a huge advocate for breastfeeding, but I think it should also be said that, after trying so hard, if for any reason you have to end up supplementing or switching to formula, PLEASE PLEASE PLEASE don't feel guilty. It sounds like you have given it a heck of a run! Ultimately, you have to do what is best for that little guy, whatever that may be.
Best of luck and hang in there, momma!
(Also- I, too, am clueless as to what a breast sheild is...)