As a log term windows admin who's cleaned up more home computers than I care to count, here are my tips:
1. Ensure windows updates are set to download and install automatically.
2. Install AVG Free, sure MS essentials is good, but I guarantee every virus is written to avoid it, I go with 3rd party AV wherever possible.
3. Install Chrome for web browsing, sync the account to google
4. Setup his account as a regular user, don't give him the admin password
5. Setup something to backup Warcraft, it's a huge download, you don't want to be doing it again if you need to reinstall
And that's it, it's basic security but win7 is pretty good, the above has been enough to keep our home XP machine safe for many years.
Ultimately it's a kids computer and they're going to click anything shiny, sooner or later it will get a virus. There are a few key points to bear in mind here:
1. It's going to happen, preventing it is pretty much impossible.
2. Your other computers are Linux, so the risk to them is negligible.
3. Most viruses these days are botnets or phishing, so long as he's not spending a fortune on a debit card, the risk to him is minimal.
4. All the software I recommended will update itself, so it's zero maintenance. That's a major factor in keeping windows secure.
5. If it does end up riddled with viruses, a quick re-install over the top, followed by a sync to google and it's all back to normal, including your files and settings.