I would make these suggestions for after this is finished and you're up and running again:
Password protect your administrative-level user account. Create a limited user account for your son. That way, anything he wants to install must pass your scrutiny and virus/malware scans first. Research the things he wants to install before installing them, as some of these game mods can be a source of infection (speaking from experience).
Make
Macrium images of your OS drive on a regular basis, (once a week or so), so if something gets in, you can easily restore an image and be back in business.
Use a layered approach to protection. As an example:
Avast! (free or paid - I'd only use the 3 web shields + software updater, nothing else) or ESET NOD32 (paid)
Malwarebytes Antimalware (MBAM) (free or paid)
Malwarebytes Antiexploit (MBAE) (free or paid)
CryptoPrevent (free or paid)
SuperAntiSpyware (SAS) (free or paid)
Firefox browser as default, with Flash and Java set to Ask to Activate, WOT plugin for browsing safety, LastPass Password Manager (never save passwords in the browsers), and another plugin which shall remain unnamed (per forum rules).
The free versions of MBAM and SAS are on-demand only, and update/run when you call them. CryptoPrevent sets group policies to prevent malware from running from known places (like appdata folders) and needs to be updated manually. MBAE protects browsers from zero-day exploits.
It's a good opportunity to teach your son about internet safety and cautions when downloading/installing things. I did this myself with my son, using these methods, over the course of a year. He then began "cleaning" out all his friends' infected computers and teaching them internet safety. :)
Hope that helps!