1. I guess you have already created a non-administrator user account for your son. If not yet, do so, and activate Parental Control from the System Preferences>>Parental Control window.
2. While your son's machine is open and connected, open System Preferences in YOUR machine. You should be able to see a third radio button with the option of managing a remote user's parental control. Select that one and hit [Continue]. You will see the Parental Control window, but now listing the users at your son's machine. Select the non-adminstrator user you created for your son in his machine and configure it as needed.
But one warning: I used to do that, managing Parental Controls on my son's MacBook Pro (OS X Lion) from my MacBook (OS X Lion). When I upgraded my laptop to OS X Mountain Lion, Parental Control stopped working. I could configure everything at my machine, as always, but when changes were supposed to have been applied, I could realize they hadn't. In fact, that's an open discussion I started in this forum, but I have received no response.
Good luck with this. I didn't go deep into the details when giving instructions for you to configure this, because I didn't want to be verbose, but if you need more detail, please just let me know. I will gladly add step-by-step instructions.