A 4-year-old boy from rural Maharashtra presents with a painless abdominal mass noticed by his mother during bathing. He is otherwise well, with no fever, weight loss, or hematuria. On examination, a firm, non-tender mass is palpable in the left flank, confined to the abdomen. Ultrasound shows a heterogeneous mass arising from the left kidney with areas of necrosis and calcification. There is no evidence of metastatic disease on chest X-ray. What is the most appropriate next step in management?