I suspect a configurable web filter such as Dan's Guardian (see http://dansguardian.org but no pfSense package) would be more suitable if you want to prevent download of copyright material.
Dan's Guardian can be configured to block downloads of particular types of files (based on the content description and/or file name extension) and can be configured to apply rules to particular users identified by IP address or login.
pfSense users seem to use squid and squidguard (see http://www.squid-cache.org and http://www.squidguard.org, pfSense packages available) for "web filtering" but this combination doesn't appear to provide a mechanism to block content such as MP3 files and movies.
I have used Dan's Guardian (which defaults to use squid) but not squid with squidguard.
If you have confidence in your son's sense of responsibility, it might be sufficient to explain the possible consequences of downloading copyrighted material and ask him to get your approval before downloading any audio or video or … files.