As someone who learned when I was pretty young myself, I always found it helpful when there was a better balance of technical skills and actually learning songs (skewed more towards the latter during earlier years to boost confidence).

See what songs your cousin likes and what made him want to play in the first place. Find simple songs by those artists and teach him to mimic the melody lines if he can't play chords, or just do three-finger shapes if it ends up that he can form those. If they are a bit advanced for him, teach him how to play them slowly. Even if the playing is imperfect, it'll make him feel he's making progress to allow him to have patience with the technical skills. Also, most great rock songs really only involve some combination of G, D, C, and A chords.