Show him Adam Neely's YouTube channel if he is not watching it already. A massive wealth of music theory knowledge and he touches on more advanced tonality topics pretty often. He also plays bass, so lots of the "vlog" and practical stuff is using bass.
Your son sounds like he might be bored by a schools traditional learning pretty quickly, channels like Adams and Andrew Huang would be great ways to keep his curiosity piqued!
Funny story: Watching "A Hard Day's Night" with my son, when he was around 8 years old: "Daddy, why are the Beatles out of tune?" (The tracks in the movie were slightly off-pitch compared to the album.)
Turns out he has perfect pitch. As in, he can start singing a song in the exact pitch he learned it -- close enough to sound in tune with the recording. This is accuracy to the discrimination level of trained musicians -- I estimate about 0.5% or 1/10 of a semi-tone.
Oddly, or not, it took him several years to learn to hear music on a relative basis -- to hear C-G as similar to D-A. Transposed songs (Sinatra sings "Something" in F) sounded to him like completely different songs.
Now he has developed an excellent musical ear, and taught himself to play bass.
relevant: https://www.pianostreet.com/smf/index.php?topic=34782.0