If he files a class action, that requires that other people "join in" and agree to give evidence or counter-sign to agreed evidence. The only people who would sensibly do that would be people who have quit the game or who want to quit the game, as a successful claim from say 1000 people would most likely bankrupt K2 and bye bye game.
Anybody who's never paid either for premium or for knight cash wouldn't be able to claim for any damages as the service provided to them was free.
Anybody who's under 18 would have an extremely difficult time making a claim, because how many of their parents are likely to support their claim? I think most parents would have a quiet giggle and find some way to placate their child "how about we go out and get a new game then?". How many kids who signed up for the game followed the TOS and got specific permission from their parents to open a game account? Any law abiding parent wouldn't stand up in court and lie about giving permission just for the sake of their kid getting $15 back from K2.
As much as we dislike the service from K2, suing them is just shooting ourselves in the foot!