Definitely check out the new Wolfram Home edition and student offerings that provide functions that can be very easily used to promote understanding of an extraordinary variety of mathematical concepts with visualization. Since your nephew, perhaps with your help, seems to understand the basics of Python, he already probably understands enough mathematical and foundational computer science to fiddle quite usefully with Mathematica, which will give him the ability to visualize/realize/understand numerous mathematical objects of his own creation and interest with minimal programming. However, once he begins to learn how to use functions together algorithmics will also become familiar, as will fundamental notions such as "function" and "function composition" and the many distinct classes of functions. As his maturity grows, he will have a tool set that he can make practical use of through college and beyond. Wolfram maintains a huge archive of code that can be easily "pasted in" and immediately executed for the student to study specific topics.
Because programming in Mathematica can be purely symbolic, many functions can work directly on seemingly "non-mathematical objects" such as images, as well as provide mathematically exact answers and not just approximations that are generally the case with languages designed for finite-state machines.
Wolfram has a new book (available online) at provides a very gentle introduction to the product and if you read it together, the combination could provide endless topics for conversation and learning. One thing is for sure, had I had a product like this available to me when I was in school, I would have learned much, much, much more mathematics, it would have been fun, and better still would be in a better position to put to good use what I have learned.
Not sure to what extent the Home and Student editions support Wolfram Alpha, but this capability could be used also to study other subject areas beyond mathematics, particular those with strong ties to applied mathematics. Alpha support can be added separately.
The only drawback is that you might start fighting over who gets to use the computer or you wind up buying a separate copy for yourself so that you too can enjoy the fun.