A 69-year-old man with COPD (FEV1 45% predicted) presents with increased breathlessness, productive green sputum and wheeze over 3 days. On examination, he is afebrile with oxygen saturation 90% on air and widespread wheeze. Chest X-ray shows no new consolidation. What is the most appropriate immediate pharmacological management in addition to increased use of his short-acting bronchodilator and controlled oxygen?