How to stand up to this jerk?

There's a guy that sits next to me in homeroom. He's a bit of a...unique character. Sometimes he's really serious and he's really smart when it comes to politics and stuff 13 year olds wouldn't know in a million years. And the next he's really goofy and silly and really, really mean and insensitive. He doesn't know that what he says actually leaves scars, he's hurt a bunch of my clmates. He's stereotypical, he has a bad temper, he's disrespectful to the teacher and sometimes I wonder if he's actually sane. Honestly, I don't want anything to do with him. And what really puts me off is no one says anything. They just wave it off or laugh it off, and I am just sitting there thinking "aren't you gonna say anything??" No one's had the courage to say anything to him. So next time he makes a snarky comment at a clmate or says something rude, what should I say? I am not afraid to speak my mind, but I really want a good comeback, something not too harsh but firm, something that would leave him speechless with nothing to say. Help me out here?!?!