A 28-year-old agricultural worker is admitted 2 hours after ingesting an organophosphate compound (parathion) in a suicide attempt. He is conscious, alert, with pinpoint pupils, profuse salivation, and bronchial secretions. Respiratory rate is 24/min with mild wheezing. He has received atropine 5 mg IV and shows good response (pupils dilating, secretions drying). What is the most appropriate next step in management?