Definitely have instruments around for him to experiment with - and let him figure them out to at least some extent instead of showing him the "right" way.
If you have a keyboard or piano in your house, let him at it as often as he'll mess with it. Josie goes in fits and starts with ours, but it Is in my studio, which is kind of out of the way in our house.
By all means play lots and lots of music, different kinds. Don't forget different kinds of ethnic music, too!
Scottish, Irish, Cuban, Middle Eastern, even Malagasy (from Madagascar)!
Follow his lead as much as you can when he's listening to music. Let him give you an instrument to play, or decide on a musical (or even non-musical) activity to do while music is playing: playing instruments, dancing, drawing, dancing w/toys, painting, knocking over blocks, whatever.
Get together with friends - your or his or both - to sing songs and generally have unstructured or loosely-structured music playtime. Maybe each parent can share a song or activity and other parents (and/or kids) can come up with other ideas on how to do different things with the songs.
See if your library has the CD's for Music Together - ours does! There are 9 different sets of songs, plus 3 different Summer collections. I like the songs and the arrangements they have, a nice mix of traditional, ethnic, and stuff they've composed just for MT, and the arrangements and recordings are generally well-done and not too cheesy to appeal to adults!
Create musical rituals, like good morning or bedtime or bathtime music, or songs that he might do with specific relatives - does either side of the family have songs they particularly like or which hold special meaning for them?
Most importantly, and i'm putting this in boldface,
make sure your DS sees the adults in his life - you and DH and any caregivers - REGULARLY making music! Sing, dance, read stories that have rhyme and meter (like Dr. Seuss or Sandra Boynton), play instruments, hum, whistle, play an instrument if you have one in the house to whatever level of ability you have! If any of you is inhibited about singing or dancing or musical play, now's the time to set that aside, since the musical example of adults who matter to him is THE most important factor in deciding where he goes - musically speaking - from here.
Good luck! Keep us posted!