What he needs is lighter fluid, not butane or gasoline.
Butane is a gas that’s stored under pressure as a liquid, there’s no way to add that to a Zippo unless you get one with a butane insert (which he doesn’t have since his has a wick)
Lighter fluid (which is basically/mostly naphtha) is a flamable liquid used in zippo type lighters.
There’s also charcoal lighter fluid which is still another flamable liquid that’s usually either mineral spirits or alcohol based.