An instrument is like a prosthesis, an extension of the body, music an extension of the mind - that's the order: hearing, movement, sound. Early child hood programs like music together, kindermusik, and musikgarten, are all based on this principle, as are all the big names - Kodaly, Orff, Gordon, Suzuki, Dalcrose, etc. Check em out, the voice is the first instrument, and usually the easiest to work with, or at least its the easiest to transition from when learning to navigate an externalized one. Whether you start at the piano or use the voice, same issues, in order to harness the power of sound to create and improvise music we need to train the inner ear. Start with that, it's actually lots of fun! (and everything is fun when you can actually do it, but that takes practice, which may or may not appear fun, but again let it unfold, be loose and easy with it, no amount of stress will ever help anything!).
When we sing we really do feel how hard it is to 'feel' the distance between notes, which is interesting, and how our sound feels 'big' and resonant and connected (it should anyways!); but on a piano, the body is now outside of us, and the sounds we produce 'feel' clearly defined by the keyboard. Nothing could be further from what is actually going on, sound is continually morphing, resonating, changing in infinitely subtle ways. See that, explore it, this can be done in many creative ways - I think the voice though is still a good place to start with this.
Many teens and adults (not usually kids!) often report that they feel disconnected from their instrument, and thus the music. And while that may be so, part of this process is about seeing that there is in fact no separation between sound, and what we hear. They are one and the same, or in other words, we are the sound; it is always right here, in our immediate aural field of awareness. When we see that, the body follows like a well trained dog. When you say sit, it sits. When you 'hear', it follows with movement. No separation, never was, never can be, see that, explore it. This process is not unlike self discovery, finding out who and what we truly are. The inner ear is where it all begins, usually!
The body resonates in different cavities, the vocal chords and various levels of resonance are like a synthesizer, a vocal modulator, explore those sensations, explore what is present there, a teacher may be required.
In your case, perhaps find popular music that meets his interests, and deconstruct it and put it back together again, melody, bass, harmony, learn to sing the melody, try to do everything by ear in the learning process, reading and theory come last (but usually are simply introduced in the process as a related issues arise).