A 28-year-old man from Delhi presents with a 10-day history of fever, headache, and abdominal discomfort. Blood culture on day 3 of illness grew Salmonella typhi susceptible to all first-line agents. He was started on ceftriaxone 2 g IV 12-hourly. On day 7 of antibiotic therapy, he remains febrile (38.5°C) with persistent abdominal pain and hepatomegaly. His repeat blood culture is sterile. Abdominal ultrasound shows no free fluid or abscess. What is the most appropriate next step in management?