A 35-year-old man presents with a 3-month history of exertional chest tightness and shortness of breath. Spirometry shows reversible airflow obstruction. He has experienced three emergency department visits in the past year for asthma exacerbations treated with oral steroids, and uses his salbutamol inhaler most days. According to the CTS asthma guideline, what is the most appropriate step-up therapy?