Ending Child purchases

I'm not entirely sure what you want to do, but there isn't a setting which defers purchases. If it's on an iPad, iPhone or iPod Touch then depending upon the iOS version on it you might have a Settings > General > Restrictions > Require Password option that you can set to 'Immediately' (the default is a 15 minute period during which your password doesn't need re-entering), and you can also turn of in-app purchases via the Restrictions screen. On a computer's iTunes there is what is described on the page that you posted from, or you can log out of your account on it via the Store > Sign Out menu option (or your id might also appear at the top left of the store pages, clicking on that might also give you a Sign Out option). If your son knows your account's password then you will also need to change that