A 72-year-old man with a history of COPD (FEV₁ 45% predicted) presents with acute worsening dyspnoea, increased sputum volume, and purulence over 3 days. He is mildly tachypnoeic with oxygen saturation 90% on room air, and chest radiograph shows no new infiltrate. According to Canadian Thoracic Society COPD guidelines, what treatment should be added to his short-acting bronchodilators?