A 32-year-old farmer from Punjab is brought to the emergency department 45 minutes after accidental exposure to an organophosphate pesticide spray. He presents with profuse salivation, bronchospasm, muscle fasciculations, and pinpoint pupils. His wife reports he was spraying crops without proper protective equipment. On examination: BP 100/65 mmHg, HR 62/min, RR 28/min with audible crackles bilaterally. Serum cholinesterase level is 25% of normal. Which of the following is the most appropriate immediate pharmacological intervention?