My girlfriend has a nine year old boy. He is quite well behaved. However, at dinnertime, this goes out the window. He is a chronic nibbler. He will take only the smallest of bites, often spitting what is in his mouth onto the floor, putting food into his pockets, onto the floor, ect. Dinner time is basically a battle of wills. Yelling doesn't work, as with other punishment. Having the idea of a delicious dessert does not work. He loves junk food and will sneak in the early morning before anyone else is awake and tank up. So, there is almost never junk food in the house.
His sister is quite the opposite. She eats and enjoys the yummy dessert afterwards.