hmmmm
If he is the last man standing, how can he logically save the world? Or more specifically, the Human race? He cannot mate, meaning that is the end of the human race. On top of that 100 years isn't nearly enough to eradicate all radioactive material that has been left behind, he would surely get sick. The planet itself is fine, it'll recover so there is no need to save that.. I don't see this story going ANYWHERE unless you add another character - which would void the "Last man standing" unless you do a play on words and bring a Woman into play and say "well its not a man".
Or you create a crazy twist and add some time machine, or some medical experiment that assists the main character in some form.
Never liked these type of stories because unless you screw around with something then there is no real goal for the user, unless it is to have fun with the time he has left