Reddit mentions of Linguistics: A Very Short Introduction

He wants to be a linguist--I think he would also enjoy the etymologies in the Oxford English Dictionary. I suggest taking him to a public library and sitting him down with a copy of the OED and going through a few definitions (penetrate would be a good start).

You've definitely got a linguist on your hands--if he also gets interested in computers and/or programming, he will have a lot of jobs waiting for him after he gets through college.

Please PM me if you want more help/advice.

Edit: He might enjoy these books:

http://www.amazon.com/History-English-Language-6th-Edition/dp/0205229395/ref=sr_1_2?ie=UTF8&qid=1377654532&sr=8-2&keywords=english+a+history

http://www.amazon.com/Linguistics-A-Very-Short-Introduction/dp/0192801481/ref=sr_1_1?ie=UTF8&qid=1377654559&sr=8-1&keywords=very+brief+introduction+linguistics

http://www.amazon.com/Professor-Madman-Insanity-English-Dictionary/dp/0060839783/ref=sr_1_1?s=books&ie=UTF8&qid=1377654605&sr=1-1&keywords=the+madman+and+the+professor

I'm excited for you, because there is so, so much good stuff out there to read about language.

Any introduction to linguistics would be a good place to start. I have in mind this sort of book, though I haven't read that one specifically. Linguistics is the science of language, and provides a very helpful framework for reading anything further.

If you're really interested, I think it's important to have a grasp on what the subfields are and their basic concepts (syntactic trees, phonemes and allophones, morphemes, etc.). In my opinion, an intro textbook is the best way to get this information outside of a classroom. And you can come back to r/linguistics or r/asklinguistics with any and all questions that come up.

Some people will recommend more "novelized" pop linguistics books, like Pinker's The Language Instinct, Words and Rules, various books by David Crystal or John McWhorter, among others. If you want to go any deeper than surface-level, those aren't the way, and honestly might be misleading, confusing, or give you the wrong impression of the current "state of the art" (especially Pinker...). Another option is Linguistics: A very short introduction (Amazon link)

There's no shortage of textbooks out there, and most likely everyone will recommend either what they had in intro themselves, or what they use to teach intro. I personally used an older edition of Akmajian, Farmer, Bickmore, Demers & Harnish.

A recent open-source intro textbook (2018)

See also this reddit thread or the sidebar recommendations.

Try Linguistics: A Very Short Introduction.

It's a pretty concise guide, but hits all the major points. Your university library probably has a copy.

As a sidenote, the OUP's Very Short Introduction series is excellent.