I would encourage your son to not attempt this. If he's a developer and needs to test Internet Explorer 8, have him check out http://browserstack.com. Aside from that, Internet Explorer 10 is going to give him a better experience online as developers move towards adopting use of the latest APIs, and standards.

If he wants the web to be useful and enjoyable, don't move backwards. If he wants his browser to be identified as Internet Explorer 8, he can change the Browser Mode by pressing F12. It will, by default, be in Internet Explorer 10, but he can easily change it to 8. This will modify the userAgent string provided by the browser, as well as much of its behavior.