A 35-year-old agricultural worker is admitted to a tertiary care centre in Delhi 2 hours after intentional ingestion of an organophosphate compound (parathion). He has been given atropine 10 mg IV and oxygen therapy. His respiratory status has improved, pupils are now dilated, and mouth is dry. However, he still exhibits persistent muscle fasciculations, weakness, and elevated creatine kinase (CK 800 U/L). What is the most appropriate next step in management?