So glad you've already found some help. Our boy Kingsley was so scared of the world, he was a horrible walker for the first year we had him, and even now, 7 years later, he'll still statue occasionally.
For him it is fear, of noises mostly. And we know when we can push him to keep going, and when he needs to just wait for a bit before continuing.
We had luck driving him (just down the street) for walks when we fist got him... that seemed to get him out of his zone so that he would just walk.