A 28-year-old agricultural worker is admitted with organophosphate poisoning. After receiving atropine 15 mg IV over 20 minutes with good response (pupils dilated, secretions dried, HR 72 bpm), he remains weak with persistent muscle fasciculations and inability to lift his head off the pillow. Respiratory rate is 26/min and SpO₂ is 94% on room air. What is the most appropriate next step?