You could have him play a simplified version of Joplin’s “The Entertainer” while you improvise some basic ragtime grooves with it, and/or play along on a mallet instrument.
I don’t think there’s a lot of multi-percussion and piano stuff out there, so I think you’re better off finding something jazzy/ragtime/etc. and just improvising along with it.
George Gershwin wrote 3 Preludes for solo piano, they aren’t too hard, you could play along with those. I’m sure there are simplified versions of his big pieces like Rhapsody in Blue for young piano students, too. You could play along with that.
Since you can read sheet music, why not create the percussion parts to whatever he's playing? It doesn't have to be advanced, just be formal about it. Compose the part, then score it. That way, you're not just improvising every time, and both you and your son have a roadmap to follow - your own duet.
I've been using the "Salsa Guide Book", it has arrangements for percussion and piano. Though not full songs to play, just enough to get the idea about the styles. It is piano oriented however, I find the piano approach to writing out scores to be useful for these percussion parts with many instruments.
Look at Steve Weiss Music. You will find thousands of pieces for percussion with piano accompaniment. I don't know your sons ability level, but all the pieces are graded for difficulty. I'm sure you will be able to find some fun stuff to play together.