Definitely sounds like you should seek the advice of a pediatric pulmonologist at your cf center, if you have one.
If there is a sterile vaporizer, I think the idea is to create a moist medium to loosen mucus and create an under girding of fluid to help the cilia move the junk to the tertiary to secondary to primary bronchi to trachea and out with coughing, huffing, clearance. However nebulizing is the primary key to the mucus removal mechanics a long with percussion of some sort; but I suspect your pulmonologist would suggest dry air, if mouth breathing is occurring due to stuffed up nasal passages, will impair the tracheal ciliary tree mechanism and increase the difficulty of expectorating the mucus.
"Normal" breathing requires inhaling through the nose to humidify and filter the air going into the lungs, so mouth breathing bypasses the natural physiological process of aerating your son's lungs. You may be new here, but this is the place with more cf brain power than any place on earth; so keep your questions coming and a legion of experts will be here with an over abundance of knowledge and first hand experience.
Peter