I agree with Birdman. Take it upon yourself to give him some extra special attention each and every day you're there (possibly with the intention of bringing him home yourself). I would personally just pull up a chair and sit near his cage (as close as I could) and just talk softly to him for maybe 10 minutes every time you are at the shelter. Make sure you are talking at him, and only him, so he knows you have come to give him special attention over all the other birds. After a while, start bringing over some food, and eat it right in front of him, while talking at him and (play it up a bit) clearly enjoying the food (fruit, veggies, some unsalted nuts would all be good). It may not be the first day or even the first week (if he's that traumatized), but eventually he WILL want some of that food you're making such a big deal about (allowing you the opportunity to slowly and calmly hand him some through the bars and give him some gentle verbal praise) and WILL finally approach you because for the past 3 weeks you've came and visited him regularly, made him feel special (whether he shows it or not) and haven't hurt him yet.
To give some perspective on how long these animals can take to start re-building trust with a human again when they have been thoroughly traumatized by prior owners- my bird took the better part of a YEAR to really start building momentum in learning to step up and exploring the apartment. It took several years to get him interested in nutritious food and to get him feeling safe enough to sit in your lap and play with a toy. Last summer, he was finally ready to start going out in public and seeing the wider world. After over
6 years in a loving home (really within the last 6 months), he has finally began allowing me to kiss his head and back (no petting still), finally earned shoulder privileges and has started enjoying crawling up my hubbies shirt and letting him handle him around the body a bit. I expect we will continue making progress with him
If you are willing to put in the time and effort, even the ones who appear 'beyond hope' will eventually come around. I also have to say, it sounds to me like you really feel a connection to this bird and have a spot in your heart for him. Have you considered, after some work with him at the shelter, adopting him yourself? Sometimes a certain individual just "speaks" to us and even if they are cowering in the back of the cage or ready to amputate a limb, we know they are our avian soulmate

There are a lot of "diamonds in the rough" floating around out there disguised as the kinds of birds no one in their right mind would want. All they want is someone willing to unconditionally love and care for them.