Don't bribe a kid to drink milk by putting anything in it. Coffee is a terrible choice, a child of 2 does not need caffeine. Your nephew can get his dairy in other ways, as mamawith2 says.
A two year old only needs 2 to 3 servings of dairy per day. Too much milk is not good for him. Coffee is definitely NOT good for him. It's a misconception that 2 year olds are babies and need lots of milk. They don't. If he drinks water, and eats some cheese and yogurt, along with protein, fruit, veggies, and grains, then don't worry about milk. If you or the grandparents are worried, you could add in a multivitamin, but if he doesn't want milk, don't force it on him. He'll be fine. Good luck.