Homecoming Horde by Robert Silverberg — Read

Haverford knew from his radio contracts he was the last man alive on Earth. His death was certain -- for the enemy numbered trillions, a --

The room was sealed as tightly as possible. Haverford had checked it for cracks, made sure the windows were caulked, and now kept constant guard. He was alone. He could never tell when the alien invaders would break through.