Internists aren't always familiar with CF and all health issues, plus some drs just plain graduated at the bottom of their class and just barely made it by. Usually Internists are pretty good, but it sounds like this one definitely wasn't a match for you. I wouldn't bother returning to him, which I doubt you will from the sounds of it. There are some great drs out there so don't give up.
If you've had the genetic testing at Ambry then you have the confidence that you are being treated for CF and don't need to worry about what anyone else thinks. If the dr. questions it, move on. Is the appt. on Dec. 5th with a CF clinic? It's important to be treated by one. I see mine every 3 mos. or more often if necessary, and just see my Internist about every 6 mos. unless something non-CF related comes up.