Don't confuse provincialisms for bad grammar. …
(1) In standard U.S. English, the preposition in the sentence above needs to be
with. In standard British English, too.
(2) Confusing
then and
than may be many things, but bad grammar is not one of them. (Using
than as a preposition instead of a conjunction would be a grammatical error.) More often than not, it is a spelling error made by people with minimal education—that is, no more than two graduate degrees in the humanities, sciences, or business.
This and similar errors were once a bar to academic advancement, but they now may be found in what passes for scholarly articles in learned journals and raw manuscripts submitted as chapters of edited volumes. Since reprimanding children—or now, alas, adults, too—for spelling errors is a forthright attack on their self-esteem, it has become a notable hate crime to do so. Thus, confusing
then and
than or
reign and
rain and
rein or
tenant and
tenet simply demonstrates that the offender has had a thoroughly modern American "education" and hence has become what people once unapologetically called an ignoramus.
(3) The term applicable to the specific situation you describe in the sentences that follow is
regionalism, not
provincialism. The latter term invariably has at least a whiff of reprobation about it, whereas the former does not. This is ultimately a matter of democracy in action, like it or not—that is, numbers matter. Twenty thousand Amish folks calling everything they don't like "English" might safely be called a provincial form of expression, but when at least a hundred million Americans call something a "fireplug" and when that word has been in accepted use for fully a century longer than "hydrant," you damn well better refer to it as a regionalism if you want to get out of the lexicographer's annual convention in one piece.
What is more, the plain fact is that
fireplug is neither a provincialism nor a regionalism. It is standard English in North America, and in British and Australian dictionaries, the only usage label attached to it is "United States." Put otherwise, your relative may or may not be rude, but he is certainly an ignoramus.
Why hasn't he signed on here at CI, I wonder. He'd feel at home.