combine multiple apple id

I set up an acct for my son when he was young under my credentials. He now is an adult, has a MAC and iphone etc and I now have a mac and ipad and iphone. I want my account back - he set up his own apple id for his mac and phone but all of his 4000 songs are still in my account. I want those to be on his account. How do I handle that? I want my apple id to be just my stuff.

Thanks