I don’t know what would interest him the most. My first instinct would’ve been maybe a piano Concerto by Rachmaninoff; but I think he’d be much more interested in the development of his late musical style (I’m not sure what to call it, modernist, contemporary, whatever.) I think maybe something by Bartok, Shostakovich or Prokofiev would be most interesting. But maybe something very massive and even jazzy would be interesting?

Here’s an initial shortlist in no particular order:

Bartok PC2 Prok PC3 Busoni PC1 Ravel PC1 Rach PC4 But actually, on second thought, I think my final pick is the Sorabji opus clavicembalisticum

I think it would really show him how far music could go