You don't need to provide a CC right away. When you go into the app store and get one of the free apps it will ask you to provide a payment method. Just pick the "none" option. As long as your getting the free apps just keep selecting the None option and you'll be fine but if you buy an app then you will have to provide a CC. A good way to get around that if you want to let your son buy stuff is to get him Itunes gift cards or let him buy his own gift cards to use. YOU need to make sure your Itunes is up to date though.
Tom's Idea is also a good one and works well as long as your son doesn't figure out your pasword