My son currently uses my apple ID on his phone with my credit card, and we have an apple music memebership, if he starts his own apple ID, can he still keep all the apple music and just use it under my account on his phone, i.e when he opens apple music he needs to type in the password for my account?
You should check out Family Sharing, which is designed to share "family's" purchases and credit card. This way, you don't need to transfer content, instead you share the content.
Some restrictions as fine print from aforementioned site:
Family Sharing requires a personal Apple ID signed in to iCloud and iTunes. Music, movies, TV shows, and books can be downloaded on up to 10 devices per account, five of which can be computers. iOS 8 or later and OS X Yosemite or later are required to set up or join a Family Sharing group and are recommended for full functionality. Not all content is eligible for Family Sharing. Content can be hidden by family members; hidden content is not available for download. Content downloaded from family members or acquired via redemption codes is not subject to Ask to Buy.