Microsoft Security Essentials is the only thing I have running on most of the Windows computers I administer (note: they're XP, not 7). I've never had any problems. Install that and don't worry too much about it. Install noscript on Firefox and tell him not to use IE; that will avoid most of the remaining problems. Let all software autoupdate as much as it wants.
You do want to do two other things. 1) Keep that install disc, and make sure the kid knows how to install Windows himself, plus install his games himself. I think WOW and probably LOL are both cloud-based saves so wiping the HDD is no issue. Reinstalling Windows is generally 1/4 the time and hassle of actually fixing a malware problem.
2) Let him know that he is only likely to get viruses doing things he shouldn't. Drive-by downloads on legit sites are rare. Drive-by-downloads on warez, gold sellers (for WOW), and porn are a lot more common. If he is going to do that stuff (you can't stop him) at least make sure he knows that those are dangerous sites. If his computer is acting funny after visiting one, and a reboot doesn't fix it, then wipe the install.