As a teen myself, I would suggest teaching him the basics of permaculture and then getting him something like Gaia's Garden. It introduces a lot of strategies and topics. Or let him read it. Some kids that age suddenly devour (like my cousin and I) whatever they can get their hands on.
Or you could read it with him and explain and/or condense tricky or complicated parts. Just some ideas.
Another potential book is Matt Powers "The Permaculture Student 1." It's more of a textbook, though, so might not be something he wants to just read, though it's well illustrated, and sections are short and sweet (sometimes a bit too short, kind of leaving one with more questions). It seems to be about Jr High level. You can read the reviews here. You might be able to get the second installment of the Permaculture Student for free (tehre's a thread about it here.)
Another option is Edible Landscaping with a Permaculture Twist, which has some fun projects and is pretty introductory with good explanations. It might be a bit above their reading level, but it's interesting and it's introductory.
I think a lot of permaculture-specific theory in books is either stuff that seems common sense (e.g., keep the highest-maintenance stuff close to home, don't kill your soil) or stuff that seems really complex, especially if you can't pair that theory with practical knowledge. (I always remember theory a lot better if I can try it out and go, oh okay, that's why.) I know you're in a small space but I would be tempted to identify specific permaculture skills/techniques that you can practice in the city and focus on those for now. Even with a patio garden you can learn a lot about growing plants and you can save your own seed etc. You can also learn foraging; if you know how to identify, collect and use wild plants or "weeds", those plants can be a very useful part of a permaculture design (fallingfruit.org is great for urban foraging and Seek is great for plant ID). Guerilla gardening is fun and can be as easy as making seed bombs or as advanced as guerilla grafting.
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