There are several very good, free Anti-virus and firewalls available that will be every bit as effective as is McAfeeSC:http://www.bleepingcomputer.com/forums/topic3616.html
I am not very partial to all-in-one security services for several reasons. First, the interface and programmes are usually bloated. Second, because the various parts are often pasted together by buying out other companies, the modules are often uneven in quality.
Spyware protection almost requires using several applications, because each company has slightly different critieria that it looks for when it scans your computer's hard drive.
Lastly, you are correct to assume that most malware requires user action to get into a computer. Encourage your son to always research any application (or source) before trusting it to be malware-free, or to discuss the download with you before he presses the magic button. Some versions of Windows allow different levels by user; consider changing his status.