I’ve taught a few 5 and 6 year olds and the best advice I can give you is to teach simplified material. Single note melodies and chords will be best. Try to get your grandson to use more than one finger for playing too. Don’t forget to explain proper form (probably every lesson) and do correct his hand(s) if needed.
So, let’s list some examples:
Single note melody for “Jingle Bells”, just the first part: “jingle bells, jingle bells, jingle all the way”
Using the thumb to strum chords, again single note chords: C or C7. Have him strum quarter notes and count for him.
Give these a shot and let me know how it goes! If you need more ideas in the future, don’t hesitate to ask 🙂