The Extraordinary Adventures of Alfred Kropp by Rick Yancey

An overaverage high school boy and his uncle enter a world of knights, swords and violence. Alfred ends up with excalibur and lets it fall into the wrong hands. It's up to him to fix that problem with the help of a decendant of a Round Table knight.
~Nelson Thain, age 14

Book Description: Through a series of dangerous and violent misadventures, teenage loser Alfred Kropp rescues King Arthur's legendary sword Excalibur from the forces of evil.