Undo it? How?
You make it sound like he's some sort of wizard who can undo anything that happened. If he is, why didn't he, in some 40-some-odd years of Trek, undo the destruction of numerous other planets - say, those destroyed in the Doomsday Machine
Imagine what a fun show would that be, Spock constantly undoing anything bad that happened to anyone...
Star Trek is filled to the gills with time travel. If they can go back and save the whales, they can go back and save Vulcan. It's not like there's no access to a starship, and Old Spock could show them how to do it. "Hey Jim, how'd you like to save my planet and your dad? All you gotta do is go back in time and phaser Nero to hell before all this gets going. Hey, he's gonna die by your hand anyway, might as well kill him and save some lives in the process, right? And, the best part, you STILL get to be cap'n and hang out with all of us!"
Unless, of course, this really isn't TOS Spock and he never did the slingshot thing and doesn't know how to time travel.
I like Cracker Jacks.