Dees,
I don't know your son, and I can't diagnoe anyone, so I will make some general comments that may or may not apply to you. But you can research and discuss with your doctors. Air trapping is typical of asthma as is trapped mucus. By the way, a low number in the small airways also is typically asthma. With or without mucus.
The asthma component is a very real part of CF that many doctors don't identify, recognize or even talk about. Which is pretty amazing since most people are on asthma medicines like bronchodilators and steroids which reduce either inflammation or swelling in their airways.
The good news is asthma is completely controllable and treatable. But you have to educate yourself in this area. Plus if your doctors don't recognize the asthma component in CF, you may have to get other opinions.
What happens in the airways frequently happen in the sinuses too. Dues to certain triggers these membranes swell in both cases. I had 100 percent blockage on one side of my sinuses -- they said probable polyp and I was scheduled for surgery which I successfully avoided. There are many things that helped me --- Nasal rinses, sprays, and the off use of Singulair. Singulair helps shrink airways in the lungs but it also works in the nasal passages. They are now doing studies on Singulair and the nasal passages. I did this 12 years ago and have never had a problem since.
Also if the sinuses are clogged, they drip down into the lungs causing asthma. Particularly at night when we don't swallow as much.
If you want to know more there are threads on asthma, inflammation, allergies, and chemical sensitivities. Use the search engine, plug in a key word and if you want to know anything more that I have written, put in my name under author.
And don't worry, there is MUCH you can do.