How do we share music from our iTunes purchases with our son who has his own Apple ID but is part of our family sharing?

Family Sharing - https://support.apple.com/HT201060 - "Family Sharing makes it easy for up to six people [different Apple IDs] in your family to share each other’s iTunes, iBooks, and App Store purchases without sharing accounts. Pay for family purchases with the same credit card and approve kids’ spending right from a parent’s device. And share photos, a family calendar, and more to help keep everyone connected."

Set up Family Sharing - https://support.apple.com/HT201088

More information - http://www.howtogeek.com/212823/share-apps-music-and-videos-with-apple-family-sh aring-on-iphone-ipad/ (Mar. 2015)

Sharing purchased content with Family Sharing - http://support.apple.com/HT201085 - "With Family Sharing, family members get immediate access to each others’ music, movies, TV shows, books, and apps, and can download them to their own devices any time they like.*" "* Not all content and content types are available in all countries. To share purchased content, all family members must use the same iTunes Store country or region. Music, movies, TV shows, and books can be downloaded on up to 10 devices per account, five of which can be computers. Apps can be downloaded to any devices the family member owns or controls."

"If you want to keep some purchases private, you can choose to hide individual items." --> Hide and unhide iTunes Store, App Store, or iBooks Store purchases - https://support.apple.com/HT201322