Do your parents already know that he's spending the day gaming, or have they been told but still not caring anyway? In the U.S., finals should be finishing now, I think, so how are his grades?
For the summer, how about getting him to go to the beach or water park with you? Watching girls in swimsuits - when I was 15 - was always a big plus...
I would ask him during dinner when your parents are there, though, so they know; otherwise, he'll probably just blow you off again.
In the end, your brother'll need to be the one who has to want to change. If all else fails, just do your own chores, tell him to do his, and then go out. Otherwise, you might get the "Why didn't you do the dishes when you have nothing else to do in the house?" speech... Perhaps give a head's up beforehand, and if it's something fun, invite your brother along. If he doesn't want to change and your parents remain clueless and unhelpful, then IMHO, there's not much you can do - unless you want to get aggressive, which to be honest, probably won't do much either.