Guest,
We have a student 12 bar autoharp that lies about the music room. Our 3 year old grandson can make lovely sounds. It wouldn't replace the guitar your son admires, but its an approachable musical instrument. We have a uke (also popular), a trumpet, a lap steel, harmonicas, penny whistles, kazoos, etc., lying about for little (and big) fingers to try in addition to the piano. Please let us know about your son's progress.

Best wishes, Chance.