encourage him to play any instrument. even if he learns to play the harp (just a random example lol) you too both will still have the change to jam some mad music
i have a brother and sister. she tried piano, he tried saxophone... i play the guitar since i was 12 so i thought: oof now we can do some nice arrangements...
they both only tried for 2 weeks and quitted lmao. so here i am, wishing i had another musician in the family