Robert Silverberg 
13th Immortal 

Dale Kesley was a simple farmer in the Iowa province of the North American empire when van Arlen found him. He’d been looking for the young man for four years and came to take him to Antarctica. Coincidentally, Dale couldn’t remember much of his life before four years before. Antarctica is the thirteenth empire set up after the world had nearly destroyed itself in a nuclear war. In order to recover and rebuild the world had agreed to a simpler life. Except Antarctica. It had erected a barrier around itself and been cut off for over four hundred years. Each empire was ruled over by an immortal, mutants from the war. In short order, trying to make his way south, Dale runs awry of both North and South American Dukes and now has a price on his head. Alone, van Arlen either dead or wounded, gone in any case, his only choice is to get to Antarctica somehow. The answers to who he is and why everyone wants him dead lie there. (Goodreads)