Our 1st adoption was a puppy mill dog - afraid of everything, but also some particular things - like the tones on the telephone (thought it was a bark collar giving tones prior to shocks) shovels, men etc.
It was 1 year before we ever heard her make a sound and we thought perhaps her voicebox was damaged by shocks. What a joyous day when she made sounds! She turned out to be a wonderful dog, but it does take time, effort and much patience. It took her a couple of months to slowly push the bed we got her from another room, into the room where we sat at night and next to the sofa. We didn't notice at first that she was moving it closer to us. It took that long for her to trust us and that we wern't going to hurt her and for her to want to be near us.
I applaud you for giving this little guy a home and a chance to discover love.