What is the title of youngest son of a duke?

The title of the youngest son of a duke is typically "Lord" followed by his first name and, if applicable, his family surname. In some cases, he may also be referred to as "the Honorable" if he holds that courtesy title. However, he does not inherit a specific title like his older brothers, who may hold the title of "marquess" or "earl" if they are the heir apparent.