At eight he is more likely to teach himself. Try having him strum G, B and E string while fretting different frets on the E string. He'll hear what sounds good together. Have him try fretting 2 strings in different places. And then any open chords on the G, B and E strings.
I think it will help him build strength, agility and toughness in his fingers. There's no law that says you have to start with the open chords.
Hopefully he gets a greater interest in guitar and the two of you have some great quality time making music.
Good Luck to you both,
Gordon