A 28-year-old man from Delhi presents with a 10-day history of fever, headache, and abdominal pain. He reports visiting a street food vendor 2 weeks ago. On examination, he is febrile (39.2°C), has a relative bradycardia, and a faint rose-red rash on the trunk. Blood culture is pending. Widal test shows O antigen titre of 1:320 and H antigen titre of 1:640. What is the most appropriate next step in management?