A 3-year-old boy from rural Maharashtra is brought to the pediatric clinic with a 6-month history of progressive dyspnea on exertion and squatting posture during play. His mother reports that he becomes cyanotic and breathless after running around. On examination, he is cyanosed, clubbed, and has a single loud S2. Chest X-ray shows a 'boot-shaped' heart with right ventricular hypertrophy. Echocardiography confirms a large VSD, right ventricular outflow tract obstruction, right ventricular hypertrophy, and an overriding aorta. What is the most appropriate immediate management for this child's acute cyanotic spell?