The little boy was unhappy because he had a long name--Bradford Underwood Timothy Charles Hoppenpop-per. He wanted to leave out some of his name, but he couldn’t, because there were names of his great grandfather’s (曾祖父的), grandfather’s, father’s and even mother’s in his name. An old man was so wise, he could read the thoughts of the unhappy boy, "Let me help you, my boy," said the old man. "Put down on a piece of paper the first letters of all your names, and you will have a new name."
The boy ran all the way home and did so at once. Then he got a new name, as short as other boys’. He smiled a big smile.
In the passage two "leave out" means "to choose".