Sorry, Aegislash is similar to a lot of Pokemon that evolve via stone, they can't learn many moves after they evolve. So, nope, can't learn Sacred Sword.
But! Not all hope is lost, you don't need to get a new Doublade, because if you have gotten far enough into the game and reached Dendemille Town, there is a house near the Pokemon centre that has a move reminder living in it. If you have a heart scale then she can teach your Aegislash the move Sacred Sword, along with a selection of other sweet moves as well.
No heart scales? Just head off to Route 8, 12, Ambrette Town, Azure Bay, or Cyllage City, so you can catch Luvdisk, they sometimes carry heart scales (don't forget you need a fishing rod), a net ball should capture Luvdisk at full health.
And if you're not near Dendemille town yet, well either stick in there or maybe a new Doublade is not such a bad idea after all.
