Also, the Amish are known sometimes as the Plain People, along with other religious groups, because of the plain ways they dress and live

Well Amish people wear plain clothes and have plain looks so I think "plain" represents the boy - who was a music prodigy.

I also think you can interpret it to that even though he didn't look like anything special - plain - he still had an amazing talent - a music prodigy.

Hope it helped or that it's a solution you can use (;