My son and I share an apple ID, which is fine, until he can read my messages on his iPod! How do I change that without him losing all the stuff he already has?

My son and I share an apple ID which was a good idea at first, but now most of my iphone messages show up on his iPod. How can I change this without him losing the items he already has purchased? I'm thinking he's gonna have to make his own student apple ID, but you can't sync to more Han one library at a time. Found that out when my laptop crashed and had to re do everything.