I think you've got some good ideas to help spark the interest in your son.
1. Enroll him in music for young children classes (which I believe you said he's already doing?)
2. Show him *your* interest in piano and music in general in a few ways: find pieces that you enjoy playing; put on music in the background, or to dance to, while you're hanging out at home; look at youtube videos of performances (my kids love this!); go to live music performances if possible.
3. Cater to his age and development by listening to and playing music geared to young children, and perhaps looking at the pictures, and trying a few basic activities, in piano methods for children such as MFPA. There are others too - Music for Young Mozarts is one I used to use ages ago before MFPA came out. You can browse books at your local shop, or even online. One site (sheetmusicplus.com) allows you "sneak peeks" inside most of the books they sell.
All of these things are great ways of turning him on to music.
In terms of more specific advice from us (re. books), if you tell us more about your musical interests (what styles, etc you like), and some books you've liked and why (and why you haven't liked other books) then we could make more specific suggestions.