In the App Store, choose the Featured tab. Scroll to the bottom of the screen where you should see your son's Apple ID. Tap on it and chose Sign out.
Once it finishes, tap it again, choose to sign in with an existing account (or some similar wording) and enter your own Apple ID and password.
If your son has been purchasing apps on your iPad, you will need his Apple ID to update them. Same process. Or delete them and buy them again with your own ID, if the prices isn't' a big factor.