Read an article about that a few years back.
That kid couldn't play a note on a banjo, or any other instrument, for that matter. He was picked because he looked like a retard.
Kid was wearing a baggy shirt and the real banjo player was sitting behind him with his arms going through the kid's shirt.
Also movie outfit flat out stole the song from the author. Author wouldn't sign off on a paltry one time royalty payout offer, rather demanded and got a per piece pay out.