A 68-year-old man with chronic obstructive pulmonary disease (COPD) presents with increased dyspnoea, sputum volume and sputum purulence over 3 days. He has a 40-pack-year smoking history. His usual medications include tiotropium and salbutamol. On examination, he is mildly tachypnoeic with diffuse wheeze but no signs of consolidation. SpO2 is 92% on room air. According to COPD management guidelines, which is the most appropriate next step?