Ask him politely to move. If this does not work you could bribe him to change places? Tell him you'll bring him in something that he likes. If this STILL doesn't work, tell your teacher and asked to be moved.
before u go to school, take an index card and write shut up on it. if u can predict what he might say back, like if he always says "you shut up" or something then get a card that says a good comeback so if he says something back, your prepared and it's kinda funny too. I did it once and it worked. the guy next to me said you shut up so I showed him a card that said no lol. he didn't talk to me for a week thank god and after that he didn't bother me