Here are some things to do to inspire your young player to read notes!
COVER UP THE TITLE to the song so they won't know what piece it is right off the bat.
Turn the book UPSIDE DOWN after he knows a piece and have him play the notes that way! It doesn't usually sound good, but he's reading!
Ask your child to start the song on THIS MEASURE RIGHT HERE (a random measure, NOT the beginning of the song.)
They have the skills to sight-read, it's up to us to make sure they USE them so they can continue to develop the skill.