Ventrilo and TeamSpeak both require servers to connect on so that you can talk to people. That usually costs money to get a "professional" solution which has high uptime. There are also some exe's of servers floating around that you can get to host your own server.
A word of caution though, starting your 12yr old on WoW is a bit like giving him his first crack pipe. Bad idea. Bad **** idea.
Have to agree, my brother has been 'hooked' for years now .. and 'hooked' is definitely the correct word to use. There is a definite link with his increase in weight, his drop in fitness level and WOW.
ventrilo and a server rental is about $10 month for 20-25 slot server. The client is free so if his friends already have a server that's all he will need. A word of caution; if the ventrilo server (or whatever they decide to use) is not password protected make sure you occassioanlly listen in. I have an 11 year old and I have been shocked to hear some of the things said even though the offending parties know there are underage kids around. Fortunately, or unfortunately for them, they did not know I was listening.
as for WoW = crack, that is a bit misleading. In no way shape or form is crack as bad for you as WoW. WoW = crack while using heroine.
I was the 15 year old with Everquest (before any expansions), and yes...set time limits for play. Many sleepless "must stay here and kill X to get X/get a new level/etc."
Wow, about 1-2 years ago, was better due to my age...but still too many "need to work tomorrow, but I'll be ok..."
As for your question, I would also recommend Ventrilo (over Teamspeak), at least while using the "pre latest expansion" Wow. My group of friends could never get Teamspeak to work correctly in the older version, but Ventrilo always worked well.