A son of a Duke, Marquess or Earl would be given what is termed a "courtesy" title, which conferred no real legal rights ( he could not sit in the House of Lords for example) but gave him a suitable family title to reflect his status, till he could succeed to the Dukedom on his father's death.

An example from life : the Duke of Devonshire's eldest son is given the courtesy title of Marquess of Hartington. He derives this title from one his family have the right to use. He would only have been allowed to sit in the House of Lords when he succeeded to his father's title, as Duke when his father died. The family name is Cavendish. Once the "new" Duke had a legitimate male heir his son wold become the next Marquess of Hartington in turn.