I had a similar thing happen here in California when my truck was just a few months old. They guy kept coming and in a fight there was no question he could take me. But in Cal-i-fornia
(as our Governor says it) we generally aren't allowed carry guns. But that gasoline nozzle makes a dandy weapon, especially if you pull out a lighter at the same time.
In your case, you politely asked the guy to stay away. When he fails the courtesy test, especially saying "It ain't no big deal, man..." it really is about to become a big deal.
He then fails the attitude test when you tell him to leave you alone and to back up. The Back-up! is not
a request, so his ignoring it says he views your words as idle threats at least... or at the most he thinks he can take you regardless.
Once he moves around the front of the car and proceeds towards the driver's door he has closed the distance to where he can attempt to rush you. Your reaction, I think was reasonable... given the time, place and circumstances.
I kind of ran through this in my mind of what a reasonable person might do if they really just wanted to look a the car. When you tell them to stay away, most people would stay a few feet from the car and sensing hostility, try to look inside thru the off-side windows. They might ask about the interior, seating capacity, handling, engine type, etc. But when they ignore repeated instructions to stay back or back off.... well, that's trouble brewin' for ya.