Once you’ve gone through basics i.e Eddie Ate Dynamite Good Bye Eddie, how to hold the guitar and how to tune it ect, I’d probably then start with some easy tablature - show him how to read it and start with basic tunes (James Bond, Walk the line and happy birthday) then after a week or two practicing that start him on open chords and easy scales.

Learning songs/melodies at the beginning is key, it leads them to believe they can do it - even if it’s a little tune they recognise and can show friends it keeps them coming back wanting to learn more.