A 42-year-old man presents to his GP with a 3-month history of intermittent wheeze and shortness of breath, worse at night and with exercise. He uses salbutamol inhaler most days. Spirometry shows FEV₁ 70% predicted with a 15% and 250 mL improvement post-bronchodilator. According to the Australian Asthma Handbook, what is the most appropriate next step in pharmacological management?