I bet you that Santa Boombox, left under the tree by Santa, hand picked by the fat guy himself "because you were such a good boy!" would absolutely slay it, Lol.
I would personally save something a lil more serious from the age of maybe 8 or 10 up, because it would mean a lil ore to him by then. OK, maybe 6 : )
My personal suggestion, there, since you have such great taste in the ecosphere why not give him the Atom?
Him being able to use their cool app from an old phone of yours or from your Room Core would be convenient.
You're a seriously cool Dad, by the way. And, for what it's worth, I was just having a conversation with my elderly Mom the other day about how she helped shape my love of music from early on and throughout my entire childhood. So yeah, it's very relevant, what you're doing, imo.