A 58-year-old man from Delhi presents to the emergency department with acute onset severe epigastric pain and abdominal rigidity for 6 hours. He has a history of peptic ulcer disease. Plain abdominal radiograph shows pneumoperitoneum with a small amount of free air under the diaphragm. His vitals are: BP 100/60 mmHg, HR 118/min, RR 22/min, Temperature 37.8°C. He is alert and oriented. What is the most appropriate immediate next step in management?