Actually, I will suggest that you go the other way. At that age, you need to start with songs or the student will lose interest (and not practice!).
Have him make a list of songs he wants to learn how to play, and teach him the basic melodies (the vocal line, for example, or the simple melody on guitar). I wouldn't even get into chord theory at first, or music reading - teach him how to PLAY some chords, just where to put his fingers, some very basic technique, but remember to teach him *songs,* something he can take away from the lesson and practice at home. At 9 years old, students can memorize things but lack deeper understanding of significance of things like scales & modes, even if they can memorize them. At this age, students are just beginning to develop critical thinking skills, and you don't want to go over his head.
And don't say that playing guitar will help him get girls - even if it's true, in my experience with students this age, he will think girls have cooties
At 9-10, he shouldn't have any trouble with fine motor skills and dexterity, so I would recommend a full-size instrument (we're talking about guitar, not DB, right? ~25" scale).
As he improves over time, you can begin to introduce him to the idea that chords are built from chord tones, which come from keys, scales, etc, and teach him those things. I would not push music reading too heavily at this point lest your student's eyes glaze over, although I would avoid teaching him tab, as it's a crutch and you don't want him getting on the internet to "learn" songs instead of doing it by ear from the recording.
My 2 cents,
"Mama" Dave, the nickname coming from years of teaching music to youngsters while working at a local music store