Yes could potentially use PFSense to throttle his speed. I know for a fact that WoW has parental controls that you can enable. I dont think the other games do.
You would have to throttle the connection to the point that is super super slow. I would hazard that you try other methods to limit his gaming time though and perhaps if you would game with him or show an interest , even if faked on your part , in his activity , you might have better luck at limiting his time?
You would have several options in PFSense to do this from using a schedule to traffic shaping to actually using the limiter.
Also how technically inclined is he as well? If he is pretty tech savvy then it will make the job that much harder. Most Xbox's and PS3's have WiFi so if your in a densely populated residential area , there would potentially an unprotected WiFi he could jump on and avoid your throttling altogether.
As a father myself , I find that by gaming with the kids and showing interest in their hobbies , it makes it easier and they are willing to live with limits on game time and have reached the point that for the most part they do it themselves without me having to tell them.
Good luck to you.