A 28-year-old man is brought to the emergency department following a high-speed motor vehicle collision. He is haemodynamically stable with a blood pressure of 110/70 mmHg and heart rate of 88 bpm. On examination, he has tenderness in the left upper quadrant with mild guarding. FAST scan shows free fluid in the left paracolic gutter. CT abdomen with IV contrast reveals a laceration of the spleen with a small amount of perisplenic fluid collection but no active extravasation. The patient remains haemodynamically stable after 2 hours of observation. What is the most appropriate next step in management?