A 78-year-old man is admitted with confusion, fever, and productive cough. Chest x-ray shows right upper lobe consolidation. He is hypotensive despite initial fluid bolus and requires norepinephrine infusion. Blood cultures grow Streptococcus pneumoniae. According to current sepsis and pneumonia guidelines, what is the most appropriate choice and duration of antibiotic therapy once susceptibilities confirm penicillin-susceptible pneumococcus and the patient is clinically improving?