You can't treat this student like a beginner, because he's not (I say "he" to keep it simple). That would be insulting. Especially at such a tender age, at which people can be abnormally self-conscious. As others have said, ask what he is hoping to learn. But don't ask for long-range plans, because even if the student has personal goals, that can be a very intrusive question for a young teenager. Besides, it's too soon for him to know if those goals are attainable, or if they should be even higher. Those issues will clarify themselves over time as he learns more. If he reads a little, I would find out whatever it is that he's just dying to play and use that. It doesn't even matter what it is. You can teach from almost any piece of music. If it's a million miles over his head, that will become clear soon enough, but even then, you can use some part of it to accomplish something. If he really can't read, I would start with something he has already composed or learned by ear. Ask him to choose something slow and straightforward, have him play it for you, and show him how to write it out. The first project or two will be slow going, and when I do this, I don't ask for all the details at once. I have them work in layers, discovering where to write the notes at first with no measures or other details until later. Maybe write in sharps and flats as they go, then "discover" the key/scale along the way. Then rhythm later, because that's easy to teach and learn. Don't worry too much about the written set-up: clefs etc, unless it moves out of normal clef ranges and you have to indicate that. For students who have already spent a lot of time improvising and making their own discoveries, I also help them write chord names above their written music, like a jazz or pop chart, because it's so likely that they'll use charts later on. I think it's helpful to let them see early on all the different ways we can express musical thoughts on paper.
Just my ideas... good luck.
Topic: What would you do with a student like this ?