A 35-year-old man from Bangalore with a history of recurrent Neisseria meningitidis infections (2 episodes in the past 18 months) presents to the clinic for evaluation. Serum complement levels show: C5 = 2% (normal 10–20%), C6 = 8% (normal 10–20%), C7 = 12% (normal 10–20%), C8 = 5% (normal 10–20%), C9 = 15% (normal 10–20%). What is the most appropriate next step in management?