My younger brother relies on my parents and me too much that he never does anything for himself?

My 15 yr old brother has spent over 3 years playing the computer game Runescape non stop everyday. After school he'll jump on the computer and won't get off. He'll eat the dinner that was cooked for him at the computer and will only get off to sleep. If he's not playing on the computer, he's watching TV and eating or playing Xbox and eating. He never goes out therefore he has developed no social skills, common sense, does not know how to behave properly in public and brings attention to himself. He is failing school and he's so indecisive on what he wants to be when he leaves school and because he's always on the computer, even I don't know his interests. I've had the talk with him, telling him to do things for himself and to get out more but he would get sensitive and angry. My parents never ground him and never follow through the punishments they give him so he gets away with many things. He's so young and I want him to be able to grow and become an independent in the future. Sorry that this is so long. But it's been bothering me for a while. Mature answers please. Thanks heaps!