Can you have an apple id just for iMessage

We have an iPad and my son has a touch. They're both under 1 apple id so when either one of us iMessages the other can read the messages. I like that I can read his (he's 9), but I don't want him to be able to read mine. Plus, he doesn't know the password so I have to approve all his downloads. How do I change it to where he can't see my messages? Thanks!