[William Cloud Hicklin]

Tolkien wrote (can't find the cite) that, since the Valar had no authority to take away the Gift of Men (death), that any person with any Mannish blood was a Man, regardless of parentage. Earendil and Elwing and and their children were an Exception granted under extraordinary circumstances, and the choice only applied to them and their children, and Elrond's children. (Note also that Mithrellas' descendants were Men).

Dior lived and died before that ruling, and in any event as observed above it wouldn't have applied, since Luthien was mortal by the time he was born. Now, what sort of lifespan he might have had since genetically he was half-Elven (or 1/4E 1/4M) and conceivably may have had a more durable hroa we don't know, but it would have been finite. The specific extended lifespans granted to the House of Elros were, again, part of the Numenorean gift package.