July recalls

A 7-day-old boy is brought in vomiting green fluid since the morning. He is lethargic but still latching, capillary refill is under 2 seconds, and he becomes very distressed whenever the abdomen is touched. He was born at term after an uncomplicated pregnancy and had been feeding without difficulty until this morning. His abdomen looks fuller than it did earlier in the day and he has not passed stool since yesterday. What is the most appropriate initial management?