I kind of consider arranger lessons as the equivalent of accordion lessons (or playing) back in the day. You can play complex tunes with difficult chords LONG before you might have the technique to play them if you were just learning piano...

One of the best things to happen to me was moving to another house after my first year of piano. While it was being built, we had no room for a piano, so my folks got me an accordion. I learned SO much in the way of understanding chord relationships (the buttons are in a cycle of fifths layout, so the same relationships meant the same jump, no matter what key you were in) that would have taken MUCH longer to learn on a piano...

BTW, if it were ME buying this, I'd remember he's EIGHT..! Used might be EXACTLY the way to go, given how destructive little boys can be!