Whether permanent damage occurs, when the lungs are exposed to a chemical irritant, depends on the intensity of the exposure, how strong and how long. What you describe is probably the equivalent of acute bronchitis and will probably not result in permanent damage, but cough and mucus could persist for some weeks. The treatment prescribed, including the prednisone, was quite appropriate.
In some instances, such as this, the chemical can precipitate asthma. Fortunately, his having Sickle Cell Trait should not be a factor in the resolution of this illness.