A 28-year-old man presents with a painless ulcer on the glans penis for 3 weeks. On examination, the ulcer has a clean base with indurated edges and regional lymphadenopathy. Dark-field microscopy of exudate shows motile spirochetes. RPR is positive at 1:32 dilution. He is allergic to penicillin (anaphylaxis). What is the most appropriate next step in management?