A 3-year-old boy from rural Maharashtra presents with progressive cyanosis since infancy, clubbing of fingers, and recurrent squatting posture during play. On examination, he is cyanosed with a single loud S2 and a systolic ejection murmur at the left sternal border. Chest X-ray shows a 'boot-shaped' heart with decreased pulmonary vascular markings. Echocardiography confirms tetralogy of Fallot (TOF) with moderate right ventricular outflow tract obstruction and a large VSD. The child is currently on no cardiac medications. What is the most appropriate next step in management?