Do these:

1. Activate Parental Control in IE. Link is in the previous post. Make sure you put a password.

2. Install Firefox and a plugin called Glubble in your son's pc then switch to child mode.

3. Install Firefox and Glubble in your PC so you can approve or deny your son's request to access the websites that are not in the whitelist.

3. Delete the Safe Mode version of Firefox

4. Uninstall the other browsers -- Chrome, Opera, etc.