Worlds in his universe.
Besides, who's to say that he didn't go off looking for a way to undo it all and just didn't tell anyone?
What makes you think that would matter enough that he would be able to turn his back on that many lives being exterminated?
As for a way prevent it, the simplest would be to use the warp speed breakaway factor to take Enterprise
back to the point in time when Nero emerges in the past and blow him to hell with a spread of 24th century quantum torpedoes. Which I'm reasonably sure Spock should be able to fabricate.