A 3-year-old boy from rural Maharashtra presents with a 6-month history of progressive dyspnea on exertion and squatting posture during play. On examination, he has a single loud S2, a systolic ejection murmur at the left upper sternal border, and cyanosis. Chest X-ray shows a 'boot-shaped' heart with right ventricular hypertrophy. Echocardiography confirms tetralogy of Fallot (TOF) with moderate right ventricular outflow tract obstruction and an unrestrictive VSD. The child is currently on no cardiac medications. What is the most appropriate immediate next step in management?