Read some background on this story. It said the kid was picked because it fit the optics the film makers wanted, but couldn't play a note on the banjo. Still can't. That's where the picture on the right is from.
They put a real baggy shirt on the kid and set up a real banjo player behind the him who put his arms into the baggy sleeves from behind.
That report also said the author of the song sued the shit out of the movie company, because they used the song without permission or rights. The movie company wanted to do a buy out but the real writer forced a royalty set up. Made bank.
Can't remember where I saw this, but it was from a result in a simple search.