I don't want to be in a band with a mentally disabled guy
That sounds harsh. Can you tell me why you don't like him?
Because he can't play music worth shit, he's making me look bad, fuck the guy
That seems like a tough situation, but maybe you're judging him too soon. If he practices some more he might turn out to be a great player. I don't think it would be right of you to kick him out of the group.
No way. He's got to move on. I'm the singer. Sometimes his playing makes my voice unbearable to listen to, seriously.
Well maybe the problem is also with you, as you even said your singing sometimes sounds bad. You should accept people for who they are. If things are not working out you should have a discussion as a band about what to do, and make sure the decision you make is fair to everyone.