There isn't much he could do. Fast and Furious appears to have been an administration attempt to whip up public support for more restrictive federal gun controls, but that not only was botched, it probably was itself criminal. Holder will be lucky to avoid prison time over it. Obama has now tied himself to it with his claim of executive privilege. In any event that was Obama's shot at this issue, and it didn't work out. He won't dare touch it again even if he is reelected.
Changing the 2nd Amendment moreover would require that congress adopt a proposed constitutional amendment doing that, and then that a supermajority of the States ratify it. Neither is going to happen regardless of who occupies the white house.