Regular nasal douching is a simple procedure which will help to keep his passages clear and seems to assist in reducing the muckiness. Quite how often will depend on each person but a minimum of once a day is recommended and more is often necessary. Young children can get used to it very easily and it should be no more of a chore than regularly brushing your teeth.
Many P.C.D. patients have found a link between dairy products and runny noses. As children need calcium to grow caution is advised when reducing dairy products in the diet. Please refer the issue to your consultant who may refer you to a dietician for more information.