Kudos to you for being involved in your son's music education.
Several fine recordings of Pictures as already mentioned. But, for me, to really understand this music the performance needs a Russian sensibility. The Valery Gergiev reading with the Vienna Philharmonic is hard to beat. I also encourage you to have your son listen to a recording of the music as originally composed; for piano. This will broaden his understanding. The Sviatoslav Richter recordings are very fine.