My 1 year old Boxer is scared of people, even the ones he knows

Question
I have a 1 year old boxer, Onyx.  He was a rescue, we got him at about 9 months.  He was with his 2 brothers, they were supposely with a breeder who couldn't care for them.  They had no human contact only the breeder shoving food under a fence.  Therefore Onyx and his brothers were very unsure of people.  One of his brothers was more confident and Onyx and the other one would follow him.
We knew that Onyx had some confidence issues and we have tried to bring him around family, friends, etc...yet he is never ok. (please note he has never shown any signs of aggression, he will jsut hide either under a table or behind a chair) He can go to my parents house everyday and he is still petrified to get out of the car and go into the house...After a few hours he is ok.  But he will not let my mom or dad pet him...They same is for boyfriends family.  I dont want to think he was abused but I dont know if his actions are a cause of it...We have had him for over 4 months now and The skittishness of him is not getting better....I was walking Onyx daily and then one day he wouldn't even take a step in the middle of the walk and turned around to go home and now wont even let me walk him to the driveway....
I would like some suggestions, they say do training courses but I cant even get him out of the car to go into a pet store and such...
I feel bad that he is so afraid and I want to be able to bring him to places that I can, but now I am finding that I dont know how he will be and I am afraid to take him...because I dont want to tramatize him because he is so scared
I want to build his confidence.  I do believe I have control over him but I think I am missing the leadership he needs...
please help
thanks
sarah

Answer
Since the emotional damage has been done and I personally recommend you turn the "breeder" in for such inhumane cruelty-- seriously....

....you will need to have friends and family bring treats for him upon greetings...

What you can do is have them entice him, moving slowly and speaking softly and gently, give him treats.

Do not take him for walks as you've done, but rather take him for a walk in front of your house back and forth never leaving your home area. This will help him be outside but at home if he feels insecure.

Do not force him to be with people he does not like or want to be with or force him to do anything.

Rather offer treats and give treats as a reward system for any positive outcome.

This will take time because he is  not socialized, abused, mistreated, and has been isolated to the point of almost being ferule.