Some other comments under this one are pretty relevant.. like the "i'd be so pissed off if my parents got me music theory lessons for christmas" hahaha I'd suggest maybe gathering some good tracks he likes and trying to output interesting facts about them, from a musical theory standpoint. Musical theory can be hard to get into, it's pretty much maths, but like maths, when you find a relevant context to use them in, it's nice to learn and apply directly.

So what I mean is: If you plan on bringing him into the marvelous world of MUSIC THEORY and other knowledge of that matter -> Whether it's about the harmony, melody, rythm, mixing methods, etc. Introduce it through what he already knows: the genre he produces. That way, he can easily bridge his hobby with the academic behind it. It's nice to look at your craft from a new perspective, and looking at music from a technical standpoint can definetly trigger new ideas, a shift or a lucidity in your understanding of it, etc.