What is the best treatment for whooping cough? My son, age 20 has it. He had a severe reaction to the baby vaccine and never had the second shot.

Azithromycin. Azithromycin is the indicated treatment for pertussis. Supportive measures such as hydration, rest and cough suppressants are also helpful but not mandatory.
Not much. Taking a macrolide antibiotic such a azithromicin helps prevent spread of the disease, but only symptomatic relief helps the cough and usually not all that well. So a prescription cough medication, honey, humidity make help.