The Knight [Book One of the Wizard Knight] by Gene Wolfe

A teenage boy passes from our world to a magical realm of seven worlds, where he is soon given the body of a mature man of heroic proportions. Forced to act as a man, inside he remains a boy - even as he sets off to find his destined sword and become a knight. His quest is filled with danger and adventure, giants, elves, dragons and a literally enchanted romance.