A 28-year-old man with a history of recurrent Neisseria meningitidis meningitis (2 episodes in 18 months) presents for evaluation. Serum complement levels are normal. Serum immunoglobulin levels are normal. Flow cytometry shows normal B and T cell counts. Specific antibody response to meningococcal polysaccharide vaccine is absent. What is the most appropriate immediate next step in management?