A 32-year-old man from rural Odisha presents with fever for 3 days, chills, and myalgia. On examination, he is febrile (39.2°C), with hepatosplenomegaly and mild jaundice. Thick and thin blood smears show P. vivax trophozoites. His haemoglobin is 11.2 g/dL, platelet count 95,000/μL, and serum creatinine 1.8 mg/dL. What is the most appropriate immediate next step in management?