Feed him one appropriately sized food item whenever he's hungry. Generally stick to something roughly the same size as the largest part of your snakes girth. You will learn to recognize when your snake is hungry with time (they way they cruise around for food, etc), until then a general guideline is every 5-7 days for neos to yearling, then every 7 days after

You will want to swtich them to rats as soon as you can, it makes things simpler