While he may not be able to make it altogether illegal, he can certainly act to oppose and prevent furtherance of ever more extreme “pro-choice” causes. Obama, for example, favored legalizing partial birth abortion and the euthanization of babies who survived failed abortions by leaving them to starve. The pro-choice lobby is not done; their agenda is far from complete. If we can’t completely reverse it, we should certainly at least stifle the damage as much as possible. Having a conservative in power definitely puts a strong hand in our favor.
As the previous poster noted, the president can appoint Supreme Court judges and other powerful officials. The difference it makes is immense.