What was the name of here eldest son?

Answer 1

Depends. Most Earls have ancestors who were promoted in the peerage, so they also have the title of Viscount or (less often) Baron. The secondary title will then be lent to the eldest son as a Courtesy Title. So the son of the Earl of Muckshire might be Viscount Mucktown - until Daddy dies and he becomes the Earl himself, and the Viscount title moves to his eldest. If there is no secondary title (rare), then the eldest son, like all the other sons, is The Honorable Fred Muck. Assuming that there is a secondary title, the courtesy title is in the form Lord (given name + family name - which is often not the same as the title). For example, Lord John Smithers, while his father would usually be referred to as, say, Lord Hirst, or very formally as Henry Harrison Hartley Earl of Hirst.