Instead of "thinking" you might have a possible co-infection by something else I hope he will do a sputum culture first to see what the infection may be. Giving antibiotics without testing for what bug and what antibiotic it would best respond to is not advisable, especially if you may be treated for MAC in the future. This is how antibiotic resistance has grown. Being an ID doc I would assume he is well aware of this - but we all know where assuming gets us.