A 49-year-old man with obesity (BMI 34 kg/m²), loud snoring and witnessed apnoeas is referred to clinic for evaluation of excessive daytime sleepiness. His Epworth Sleepiness Scale score is 15. Examination reveals a crowded oropharynx and neck circumference of 44 cm. There is no significant cardiorespiratory disease. What is the most appropriate initial management step if polysomnography confirms moderate obstructive sleep apnoea?