Sounds like classic puppy mill symptoms. Puppy mills do not handle their dogs. They feed and water and that's it.
The dogs are givein no human contact at all, so it is not surprising that he will not come to you willingly. Technically it's neglect, but I consider it a form of abuse. If the dog had been hit, he would be hand-shy.
You may want to google how to acclimate a puppy mill rescue, because you can't force socialization. The best thing you can do for this poor boy is ignore him completely. Put his crate in a high traffic area of your home and let him watch you as you go about your business.
He will come around in his own time, as he gets more and more curious about you. Don't expect miracles, however. He may never be a normal dog.
Abuse, neglect......probably considering his condition. But, if his temperment is stable he will come around. Three weeks is not unusual for a puppy who has been 'ignored'.
Which I believe is the case. He simply was left to his own devices. He wasn't groomed so he became matted which means the previous owners did nothing with him.
First, stop cornering. This escalates the stress and anxiety. And, stop the frustration.
The dog is picking up your frustration. Which means the dog is going to take more time to become used to you. I have fostered MANY of these kind of dogs.
I leave them alone. It has taken 6 weeks for some of my foster dogs to come around. All of the sudden, the dog will greet me at the door, try and get on the couch to be on my lap.
The less YOU push the better. It is a matter of building trust and that takes a whole lot of patience and NO frustration. Working one on one is probably not going to get you much.
The fact he is playing with your other dog is a big plus. I recently adopted a fearful German Shepherd mix. She would not go near my husband for almost 3 months.
I could barely touch her. I gave her space and she came to me when she was ready. She also greets my husband at the door now and tries to get in his lap.
Patience, more time and CALM.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.