13 month old New to Us, Skittish now won't come out of hiding

I recently got a dog from a breeder, they were going to use as breeding stock but changed their minds. He's never been in a house, car on a leash etc. He is 13 months old and spent all his time in a dog pen with all of the other male dogs. He darts at the slightest movement even if not towards him. We had him to the point he would come to us even though he didn't really know a name, let us pet him and would not totally startle when we moved. Now he won't eat and wont come out of his crate or hides under the end table. He wont come to us and wont allow us to pet him. What if anything can we do, or should we just return him to this breeder who has had him since he was a puppy? What can we do if anything to get him to trust us? Any ideas? He is an unneutered chocolate heeler.

Thanks!

Wynette