Some toddlers greedily gulp whole cow's milk right off the bat. But because milk has a different texture, taste, and even temperature than breast milk or formula, some kids are hesitant to make the switch. If that's the case for your toddler, try mixing whole milk with some breast milk or formula at first (say, one part whole milk and three parts of his usual stuff). Then slowly shift the ratio until he's drinking 100 percent whole milk.
You might also try giving him just a little bit of whole milk at a time a few tablespoons as a treat until he asks for more. Or mix it with some cereal. And remember, just because your toddler is drinking his own milk from a cup now doesn't mean that you need to stop nursing him. Just make sure that milk isn't his primary source of nourishment. He's grown to where he needs more.
If your child drinks more than two to three glasses of milk during the day, he may not have room at dinner for the other foods that he needs to round out his diet. (He needs plenty of iron-rich foods like leafy green vegetables and meat, for example, because milk, unlike formula, does not contain iron.) So if your toddler's thirsty, get him in the habit of drinking water throughout the day, too.