A 58-year-old man with known COPD (FEV1 45 percent predicted) presents with 3 days of increased dyspnoea, more frequent salbutamol use, and increased sputum volume and purulence. He is afebrile, oxygen saturation is 93 percent on room air, and there are no signs of pneumonia on examination. According to COPD guidelines, what is the most appropriate antibiotic decision?