I'm sure your son will tell you all the great things about WoW. WoW's been in our house for years, and before that it's been all of the earlier online multiplayer role-playing games. So I'm going to tell you what I don't like about it. (And I'm a non-player.)
My husband has played off and on since it came out (6 years ago, I think). With high supervision, kids can play. I agree that 15 should be the min age. That being said, my DH and DD8 made a character together and she just wanders around picking flowers, mining, flying from here to there. That's only because WoW and the like are DH's hobby and she's been exposed to them since she was a baby, so she's allowed to get her feet wet with high supervision.
Upside - You join a server (you need to be on the same server as your friends), meet up with your friends, form a guild, then gain experience points/levels by killing things and/or doing quests. It's a fun interactive environment where you need to work with your friends to get things done.
Downside - you need to play as much as your friends do. Otherwise they'll level up past you and you'll be no fun to play with. (so...if they play 3 hours per night and you only allow 1 hour your DS's friends will be 3x better than him pretty quick.) Then he won't be able to keep up and they'll probably dump him or be doing lots of things he's not capable of doing.
Downside - you need to play when your friends do. After school, Saturday afternoon, whenever. If they play before homework, or during dinner, you'll be expected to also.
Downside - there are people on there that you don't want to know. My DH (40+) will quite often lose his temper at some idiot. These can be kids just like your son, 22 year old weirdos, or 40-60 year old semi-adults

. DH says the mean age is 16-28 or so. You can put filters in place to keep some of the profanity and inappropriate language out, but only some. They are public channels. Everything's there. Adults do not clean up their speech. And that's just speech. There's nothing saying that they can't act like mean jerks as well.
Downside - my DH introduced his younger cousin to it. She had a super hard time controlling her time commitment and it caused problems with her college studies.
Deb