A 30 year old person met with a roadside accident. On admision his pulse rate was 120/minute, BP was 100/60 mmHg. Ultrasonagraphy examination revealed laceration of the lower pole of spleen and haemoperitoneum. He was resuscitated with blood and fluid. Two hours later, his pulse was 84/minute and BP was 120/70 mm Hg. The most appropriate course of management in this case would be-