How to deal with a condescending person?

I have a friend who, when I'm chatting online with him, always uses "..." when he's talking, and it's really getting on my nerves. It's like he's calling me stupid without really saying so. I say something like, "hey, when were the Nuremberg Trials held?", and he ALWAYS responds with something like, "...1945 and 1946...shouldn't you know that by now..". it's not just when i'm asking questions either. i'll say, "hey, i have to go to dinner", and he'll say, "ok...i guess so...". he's VERY condescending, and acts like everything you say is stupid. have you ever experienced someone like this, and how did you deal with them?