"lord" is the appropriate way to address the son of a duke if we're gonna be accurate. "young master" is appropriate too, but lord is what u would respectfully call someone who is technically of a higher status than u, which abel is compared to aisha since she hasn't married hades yet.
earl is also fitting because a title that is bestowed upon the child of a duke is usually one step down, aka an earl/viscount/baron, though in this case abel is seen as illegitimate by many so i dont think he was bestowed any title just yet (he is 10...). Not only is it weird it sounds awkward and probably isn't the right translation to use.
either way there's ur lesson on some history
the end