A 32-year-old farmer from Punjab presents to the emergency department 45 minutes after accidental exposure to an organophosphate pesticide (malathion) while spraying crops. He is diaphoretic, tremulous, and complains of blurred vision and chest tightness. Vital signs: HR 52/min, BP 110/70 mmHg, RR 24/min. Examination reveals pinpoint pupils, bronchospasm with audible crackles, and visible fasciculations in the limbs. Serum cholinesterase activity is 25% of normal. What is the immediate next step in management?