From what Jordan's mom posted, it is correct that dairy products do not "increase" mucous production, but it has been studied and proven that they can thicken secretions of mucous in individuals. This affects many people, alot without CF and many with CF. Most non CFers notice that milk creates thicker mucous secretions when they have a cold and mucous production is already "more" and "thicker", other than that a non Cfer probably doesn't pay that much mind to it. Many CFers are lactose intolerant (has inpart to do with the enzyme/digestive issue) and any amount of dairy bothers them, not by producing "more" mucous because that is not the case, but by making the already somewhat thick CF Mucous even thicker. And even those CFers who are NOT lactose intolerant, will notice thicker mucous after drinking a class of milk, eating yogurt...
My husband loves dairy too much to avoid it, but in his younger years up until about 18-19, he did for the most part completely avoid dairy unless it was part of a meal and unavoidable, and when it was unavoidable, he took lactose intolerant pills. Now that he is older, his body seems to handle the dairy a little bit better but we still limit the dairy to as little as possible so he avoids the thick mucous.
Just a little advice and personal experience.
Another idea, why don't you do a trial of taking your son off of the dairy products, for a week, two weeks or something and see if you notice a difference.
Julie