My fiance doesn't understand the social niceties of speaking with others. How do I deal with this?

Either you accept your fiance as he is and marry him or you don't. If he is embarring to you then he is not the man for you. One should not go into a marriage expecting the other person to change. If you want to marry this man then make sure you can accept him as he is, with ALL of his faults. If you can't then you have no business marrying him because you would basically be lying when you take your vows. YOU have some major decisions to make.