Q: How do I share Apple music with my son's iPhone?

Family Sharing is only for media that you own (purchased from iTunes Store).  Apple Music is a streaming music service.  Downloads from Apple Music are subscribed songs, not owned.  They are only playable on computers and devices associated with your Apple ID (the Apple Music subscriber).

Up to 6 People can make use of Apple Music through Family Sharing.  if you activated the Free Trial for Apple Music Family Membership, then up to 6 people who are part of your Family Group will have access to Apple Music.

Apple Music and Family Sharing - Apple Support

My mistake.  IF the Apple Music subscription is the Family Membership, you can share it.  But not if it's an individual membership.