How do you control what children eat?

My wife's son recently moved in with us after being away for 8 years. His eating habits at his previous home consisted of anything from McDonalds to pizza. Since he's been here, his appetite has been less than good. He's 11 and at that age, loves junk food, but when real nutritious food is given to him, he'll do anything or make up any excuse not to eat it. The food isn't THAT bad since my wife and I eat healthy. What can we do to get him to eat the right foods?