And no, his father didn't "help" him with it. Even at eight, Wolfgang was a more accomplished musician than his father. A witness to some of Mozart's feats in London:
Daines Barrington who, fascinated by Mozart’s presence in London, compiled a report for the scientific Royal Society after setting the eight-year-old an apparently impossible musical task. The child “showed the most extraordinary readiness of invention”, read Barrington’s report; in singing an unknown Italian duet and simultaneously sight-reading the orchestral parts at the harpsichord alongside his father, Mozart not only “did full justice to the duet” but also pointed out and corrected his father’s mistakes.Now sight-singing an unknown melody while simultaneously sight-reading the accompaniment on keyboard is not an impossible task: in fact it is an element of musical training found in a number of methods including Paul Hindemith's Elementary Training for Musicians. But if the accompaniment is orchestral parts that you have to reduce at sight AND if you are just eight years old, yes, that's pretty impressive! Mind you, he did even more astonishing things on a trip to Italy a few years later. He transcribed, from memory, a whole complex piece for eight voices by Allegri, after hearing it once. The piece is about ten minutes long. Now that is the most extraordinary feat of musicianship I have ever heard of. I'm sure they left it out of the movie Amadeus because it would just not be believable!
Now let's hear that other concert aria, composed when he was nine years old. This is "Conservati fedele" for soprano and orchestra:
No, I don't think there has ever been a musical genius to equal Mozart. It is a very good thing he started so young, because he only lived for thirty-five years...