A 32-year-old man from rural Maharashtra presents to the emergency department with acute onset vomiting, profuse watery diarrhoea, and severe abdominal pain 2 hours after consuming contaminated groundwater. On examination, he is hypotensive (BP 90/60 mmHg), tachycardic (HR 120/min), and has signs of dehydration. His wife, who drank the same water, is asymptomatic. Serum electrolytes show hypokalaemia and hyponatraemia. What is the most appropriate immediate next step in management?