What's causing this patient's resistant hypertension?

Based on the likely diagnosis, what should he be prescribed?
headache man

A 54-year-old male presents with fatigue, lethargy, and frequent headaches. 

He has a one-year history of hypertension that has been resistant to treatment, and, on examination, his blood pressure is 145/82 mmHg. 

Laboratory tests reveal a serum potassium level of 3.1 mmol/L.