German Shepherd won't bark

by Dusty's family
(Texas)

We adopted a GSD from a rescue shelter about a month ago. He was abandoned along the side of the road and his age is estimated to be either 2 or 4 years old. He's a great companion but he will not bark. He is capable of barking because we heard him bark at a cat once.

We want to teach him the "speak" command but nothing will make him bark so we can teach him the association. Any ideas??? Thanks!