While you wait for a diagnosis, the only other thing I do, for my son who does have CF, is give him percussions every morning and every evening, to help loosen and move the mucus out of his lungs. I think any small child would benefit from percussions if they were fighting any kind of lung infection. I think there are video posts that show how to do it... it's pretty straight forward. Maybe you could ask a physiotherapist to show you the method.
Oh... there is supplementation of the fat soluble vitamins as well... A,D, E, and K. You could read up on these vitamins and what foods have them to feed him.
Of course, I also make every attempt to feed my son well, to keep his immune system strong, so he can fight off bacteria and viruses. Almost no refined sugars, very little dairy, lots of veggies, freshly ground flax seed, and probiotics to name a few practices.
Good luck... never back down!