Are you being referred to a CF clinic/specialist? They may want to do a culture of his mucus to see what's growing prior to prescribing any abx. Maybe you could see about getting a CF sputum culture done prior to your appointment to see if he's growing any CF bugs -- MRSA, pseudomonas, steno maltophilia, h.flu...
Maybe you could contact the pulmonologist to see if they can get him in sooner being that he's developed a cough/cold....