A 34-year-old man from Mumbai with mild persistent asthma and seasonal rhinitis is prescribed montelukast 10 mg once daily at bedtime. After 3 weeks of therapy, he reports improved daytime symptoms and reduced nighttime awakening. However, his wife reports a marked change in his behavior—increased irritability, mood swings, and vivid nightmares. He denies suicidal ideation but admits feeling 'emotionally unstable.' What is the most appropriate next step in management?