Do you know for a fact that he was abused or could it have been that he had no socialization as a very young pup?
For a while, I'd just spend time, you and him getting to know each other and getting some more trust going. He needs to trust you before he can go on to trust others. Personally, I would keep him at a distance from other people or dogs or whatever particularly sets him off....a distance where he is relatively comfortable. Make it a point. Find a place where no one will appear suddenly around a corner. Keep him where he can see people, but at a distance where he is calm and fairly confident. When he sees people but is not acting afraid, give him tiny, pea sized treats...good treats...(real meat, cheese,hot dog tid bits) and praise him. Don't get closer to people until at that distance he is doing very well. Then gradually, and I mean gradually decrease the distance between the stimuli and him. If he looks terrified, don't fuss over him, no big deal, no particular attention. (that can cause a learned helplessness in dogs...they find out that it's a good and rewarding state to be in...kind of screwy but it happens)...just turn him sideways and get him to look at you. (another exercise to practice at home)
I don't know just how freaked out this dog is, so if he's super nervous, I'd just work with him at home for a time....just the two of you rather than venturing out. He's not going to get anything out of a walk if he's terrified. He's not going to associate a walk with a good time. so, you might need to put those walks off for a while.
After a while, if he's doing much better....if you have a friend come over, which might be better than taking him out on a walk where you don't know what or who you'll run into....have a friend sit down and don't pay any attention to the dog....no staring at, or bending over to try and pat the dog. These are often intimidating body movements to a dog.
As far as patting, dogs don't necessarily love patting. Most dogs get conditioned to like it because it's associated with good things, but it's not a particularly natural thing and a shy dog isn't going to just LOVE being petted all the time, especially on top of the head. So, when you want to give the dog what he likes, don't automatically assume it's patting. See what kind of food is his favorite. If he does enjoy some gentle stroking when you're just hanging out on the couch...great. But watch the top of his head. Most dogs dislike that, especially the pat, pat, pat....slappy thing people do. That is actually quite the punisher.
So, if the friend and you are sitting down chatting, just ignore the dog if he seems fearful. Quiet, low voices, slow movements, but don't walk on eggshells either....just a happy medium. If he seems comfortable in the least, have your friend toss a treat on the floor for him. Still, no attempts to approach or entice the dog to come for a "pat." Let your dog decide on his own terms and time line. I'd practice this with the same friend for a few times and another person after that. You'll have to ask some people to help you out.
For fun and exercise, for a little while, I'd stay close to home...maybe some walks where you know it's fairly quiet and he'll feel safe, and yet where he might see another person or two from somewhat of a distance. See if he'll play with a toy out on the grass.
Remember, every time he sees a person or dog or anything that is potentially scary, give him a treat and a "goooooood boy." But...only when he's not too frightened. And if he is frightened, you need to go back to where he was pretty comfortable and work up gradually. Try to associate scary things with good things. (high value treats and low key praise)
All training is motivation and reward...no force, no pulling or scolding....nothing scary, as I can see you're doing already. You can use a light weight chain for a leash if it doesn't frighten him. I would think leather is pretty tough. I have a leather leash and I let my Doberman play with it for a few seconds for a reward because he loves fooling around with it. He lets go when I tell him, "enough" though.
If you have a fenced area or even inside the house, it's a good place to work on his recall. He won't likely develop a good recall until he develops complete trust in you, which can take some time. But anytime he comes near you...even not all the way, toss him a treat and praise him. Or a favorite toy if he likes toys. Make coming to you, even on his onw accord the best thing ever. I'd hand feed him his food for a while. Can he sit? Ask for a sit and give him a handful of kibble. See if you can entice him in a playful way to follow you down the hall. Give him a handful of his dinner. Try to involve him in his "livlihood." In other words, if he wants to eat, he is going to have to trust you and follow you and look up to you to take care of him.
If you see him doing something you don't like, don't scold or say, "no." Just give him an alternative thing to do and praise/treat for that. Most dogs value food very highly and it's a great reinforcer.
You can teach him "enough." When he's chewing on his leash and he stops for even a second, say, "enough" and give him a treat. You can also give him an alternative to holding his leash in his mouth by replacing it with something else similar. Let him carry it along when he walks and praise him....a toy or a rope toy. Making the walk fun for him is really important. Don't worry too much about a few bad habits like that. The main thing is that he bonds with you and learns to trust you even more...then he'll probably start to trust a friend of yours, then another friend, then one person he sees on the road at a distance, then at a closer distance and so on. I would definitely start at home and delay the walks in the "scary" places for a while. Take baby steps. It will take time for sure.
Kudos to you for taking him on and giving him the love he deserves. I hope this helps. I'm sure someone else will have some ideas for you also.
