About 20 years ago, I took in and abused Cockatiel (broken wing never treated by the previous owner) and his mate. I had two at the time, and the abused bird was so afraid of me and my husband that he freaked everytime we got close to him. Over the years, we lost three of the four birds, and the one left is the abused one. He has been a lone bird in our house for about three years, and he is just now getting to the point of tolerating the humans in the house. He will sit on a stick that I'm holding, but will not sit on my finger. He recognizes my voice and calls for me when he hears me. He whistles with me. But I think that for my bird, this is the best it is going to get. As sad as it is, it takes a lot of patience. When your bird becomes an only child, it will respond more quickly but but by no means will it be fast. Help your bird to associate you with food, sweet talk and some attention and your relationship with him will get better.
There is an organization here in Colorado called The Gabriel Foundation that might be able to give some advice. Their website is:
http://www.thegabrielfoundation.org