I would definitely start by reserving his IP in the DHCP reservation list or setting his IP statically just below the start of the DHCP( example: 192.168.0.xx where xx is any number between 2-99 when DHCP starts at 100). DHCP reservation is the preferred method.
Second you are going to want to setup an access control for the destination IP of the server he is logged into. This will be decided on steps 4 and 5 of the Access control setup wizard on the Advanced Tab.
For reference, you should probably have him log into his server and alt tab out of the game. Open up command prompt via Start -> Run and type in CMD for XP or Windows logo -> Search -> Type Command and open up command prompt.
In command type in: Netstat and it should provide you with the IP of the server he is connected to.
http://www.sourcepeek.com/wiki/World_of_Warcraft_Realm_List Here is an older list of of server IP's. The authentication IP of the login server is probably us.login.worldofwarcraft.com or 12.129.242.22
Now you should block each server he plays on and the login server. What this should do is boot him from his session and said time and then prevent him from logging back into the game.
Blocking Ports for WOW should do the same, however if he is a smart kid, he can figure out how to force wow to use different ports. By blocking the destination IP's you take the source away.
A dlink Tech should chime in on the ability for the rule to shut down the connection or just prevent new ones. My understanding is that it will shut down the connection at said times for the IP's or Ports suggested in the rule. You can also add schedules under the tools tab and then edit the rule to reflect them.
Rundown:
Step 1: Create a schedule, if needed under the tools tab and then name the access control policy under the advanced tab. And click next.
Step 2: Select a pre-created schedule or Always And click next.
Step 3: Enter the Address Type (I reccomend MAC address because it allows the rule to work on his computer no matter his IP. Or you can reserve his IP in the DHCP list and use IP filtering.) Click OK and you should see your Machine/IP address listed at the bottom(Note you can have multiple devices or addresses here). Click Next.
Step 4: Choose block Some access and apply Web filtering. Click Next.
Step 5: Name a rule and enter either us.logon.worldofwarcraft.com or 12.129.242.22(i believe is the actual address) and click save. Then enter the IP address and name of each server he may play on. Make sure to check the enable box and using any Protocol should work. If not set the TCP/udp port to 3724 start/end. Save settings and click next.
Step 6: Enable access logging and click next.
You should now be able to keep your son from accessing Wow during specific scheduled times. It should also keep a log of access attempts should he attempt to access wow during non-approved times.
I hope this helps.