I have 2 iphones. How do I get my music on both?

My son has a Iphone5C and I have an Iphone 5. We both share the same apple store login for purchasing music and apps. I have some songs I have acquired over the years with various CDs of my own. I have those songs on my iphone as well as my itunes music. Now that my son got this new iphone 5C itunes will not allow him to add my songs to his iphone. Yet, it did before on his ipod. How can I put these songs on his iphone as well? It seems ridiculous that he should have to rebuy the songs that I already rightfully own (non itunes songs). Thanks so much!