I suggest the book
"The cartoon guide to statistics".
https://www.amazon.com/dp/0062731025/ref=cm_sw_r_cp_awdb_imm_FMKDXA7W4M9P6RTX69R1
It follows the same format as intro stat books but it's actually fun to read.
It is my optional book for my stats students. if you try, you can get a PDF of it online for free.
As far as "best" I can't say. It also depends on what in statistics you want to learn, but I agree with /u/solkim that probability and stats go hand in hand and if you want a good grounding in statistics you will also need a good grounding in probability. Having said all of this, and as silly as it may sound, the Cartoon Guide to Statistics is actually quite good at helping to understand and learn statistics (and probability) concepts.
For "books written for kids", I was a child prodigy and I liked the cartoon guides - I read the stats and the physics ones. I liked the how did we find out series as well, I think?
Textbooks might not be so bad. You find them dry, but he might not.
Emailing professors in the area was very helpful. If he understands what he's learned from the differential and integral calculus textbook, he's probably ready to talk to professors.
You don't necessarily need to teach him to follow in anyone's footsteps. If he's reading textbooks for fun, he's probably enjoys doing that. It's more important that he keep doing what he enjoys than that he imitates someone else who was successful. Still, it's nice to know about people who were similar to you in history! But it's not like he needs to go into physics or math to take advantage of his genius - some former child prodigies are already working in those fields, and while I like to think most are doing good work and advancing the state of their fields, none of them have revolutionized them recently.
Don't walk, run to the....cartoon guide to stats. No joke! Saved my bacon in grad school. Not sure how it will look as a sited work.
statistics is magic for muggles :)
i've really enjoyed reading some of the "cartoon guide" series. Here's a link to the one on stats:
https://www.amazon.com/Cartoon-Guide-Statistics-Larry-Gonick/dp/0062731025
The Cartoon Guide to Statistics is a little cheesy, but might be helpful as you think of ways to translate technical information to non-technical audiences.
Not knowing anything about your background, I would recommend this book as a place to start. $12 from Amazon. Most stat books can be a little dry ...
Wikipedia is always a good place to start since I assume you don’t just have a stats book lying around. It gives the explanation of how the math works and the formulas involved. If you’re interested in going deeper hit the citations section.
Qualtrix has a summary explanation of the math and a calculator so you can play around with it yourself.
If you want to learn more about stats and your major didn’t require at least a few classes in stats then Cartoon Guide to Statistics is legitimately a good visual explanation of stats principles for beginners. It doesn’t deep dive but it covers most of the bases in a way that’s easy to understand.
And finally, if you’re really into this now and do want to deep dive then Springer All of Statistics will get you there.
Obviously there’s more resources but these are good jumping off points.
Not exactly an answer to your question, but I found the book, The Cartoon Guide to Statistics (authored by one of my Temple Statistics Professors, Woollcott Smith), to be both helpful and entertaining.
https://www.amazon.com/Cartoon-Guide-Statistics-Larry-Gonick/dp/0062731025
You could always just send him this book.
I have taught 101 out of Freedman, Pisani, and Purves and I can testify that it is a wonderful book, so wonderful that most "telephone book sized pablum" textbooks from most publishers copy parts of it. It is not to everyone's taste but I still send people to it for clarification of points they misunderstand. Chapters 1 and 2 about controlled experiments and observational studies and Chapter 29 about tests of significance should be read by everyone who teaches statistics and by everyone who uses statistics. Also notable is the explanation of regression toward the mean in chapters 8, 9, and 10.
This is the only low level statistics book that shows any sign of thought on the part of the authors. All the rest (even Cartoon Guide to Statistics just shovel a mish mash of formulas at the student. It is the only intro statistics book that one can claim to be well written. Actually, I take that back. I can think of at least one other: Moore and McCabe also had revolutionary bits when it was written, but it is a mix of original ideas with the shovel formulas at them school.
edit; should have emphasized that it is for a 101 "statistics for poets" course. At Berkeley they supplemented it with other material when they used it for the "statistics for science majors" course.
This is great.