When I want to tell a boy that he should eat up what he is eating now and not leave anything at all, what words can I say to him? “Don’t leave food,” is this proper words? What I like to know is if this word, leave, can be used about food.
Yes. You could say:
Son, eat all of your food. Do not leave any (food) on your plate.
As mentioned in another answer, "Don't leave food on your plate" is a common phrase.
Other options include
Don't waste food.
Clean your plate.
Finish your dinner.