A 23-year-old man with a history of asthma presents with daytime wheeze and chest tightness three times per week, waking with symptoms twice per month and using his salbutamol inhaler on three days per week. He has had no emergency visits or oral steroid use in the past year. Spirometry confirms reversible airflow obstruction. He is currently using only a salbutamol metered-dose inhaler as needed. According to the Canadian Thoracic Society guidance on mild asthma, what is the most appropriate controller strategy?