It sounds like he is probably very confident in English and able to communicate well. If that's the case, I'd work with him on the things that most students need to improve in both their L1 and L2 at that age - academic English and vocabulary. How about asking him what he is interested in (like a sport, animal, celebrity, movie genre etc) and working with him to research the topic and make a non-fiction book or website about it. If he's more into fiction and creativity, you could work with him to write a short story, comic book, movie script, etc. Focus on the message or information that he wants to convey, and use grammar and vocabulary as a way to allow him to express that message more accurately, rather than as an end in itself. You could work with him to look at examples of the type of writing he is looking at, identify the features of that writing and what makes it successful. Come up with some strategies for working out unknown words and identifying new grammar patterns. You could also show him how to draft, redraft, edit, and refine his writing until it has the same characteristics as the successful examples you looked at. Don't let him fall back on the vocabulary and speech patterns that he is used to - push him to try more complex structures and use new words.
You can have a lot of fun with this level, so go for it! Good luck!