Also by 18 months of age, most children have shifted from getting a majority of their calories in a liquid form to getting it from the food that they eat. For this and other reasons it is recommended that children try to stop using the bottle by 18 months. My advice to you at this point, presuming your child is healthy and not underweight, is to try to stop the bottle and formula, and allow him to start to increase his intake of food during the day at his meals. You should then try to give him small amounts of cow's milk in a cup at each of his meals. You can also give him milk on his cereal, which is the way that many children begin to drink it.
A major reason for encouraging milk in children is because it has Vitamin D and calcium which children need large amounts of. You can get calcium from other milk products such as cheese, yogurt, and ice cream, as well as from certain vegetables such as broccoli. Vitamin D is not present in those other milk products, however your child can get Vitamin D from other foods at this time, as well as from exposure to sun light.