You could always say "Thank you for sharing, your opinion's been noted so you can stop repeating it now," nice and calmly. Sometimes ignoring things doesn't work, because they really want a reaction and will keep going until they get one. So, give him a reaction that acknowledges him but doesn't give him the reaction he wants - upset emotions from you. So just acknowledge his comment nice and calmly, but don't antagonize him; don't show anger, annoyance, frustration, hurt, sarcasm, nothing. Just nice and calm. He may very well keep on going with new things, just say thanks for sharing.

That said, talk to a school counsellor about it. When there, don't just talk about what he says to you - share that he does this to everyone and that you think he probably needs some help. If your teacher isn't addressing the problem (and for all you know, maybe he is trying to - I teach grade 4 and I never address the problems openly, so often the students who are being upset don't see that I'm trying to solve things... especially as it's often hard to change someone's behaviour immediately) then you can take it ouside of the classroom to other adults in the building. It's not acceptable for this one student to be making the classroom hostile.