Someone help me

You'd think life would happen in a different way, but it doesn't. I was at work on Saturday, when someone came in and said they heard a little kitten crying in the bushes right outside the front door,.....of course you know I was out there in a flash. Sure enough,....the little cry was loud, and I started to mew back in my kitty-call, and the little thing sure enough responded! Came right out,....I put the frightened, shaking, LITTLE thing in a box, and ran home and put him(her) in our bathroom, with a litterbox, three little kittybeds, and a small plate of food. I had to finish out my day at work, and came home to the little thing sleeping and full. I kept going in there every hour, to let him get used to me, and know I won't hurt him, but he was still so scared. Everytime I reached for him he would hiss, and try to literally climb up the wall. I felt so bad. So I left him alone for a couple more hours, and went in and put him in a towel, and cooed and petted him till he feel asleep on my chest. So adorable. I then put him on our chaise in the living room, wrapped in the towel, and watched him sleep for awhile longer. I kept him in the bathroom thinking it would be the safest place,.....I went to work on Saturday, and halfway thru the day, my husband called and said he couldn't find the little one. He looked all thru the bathroom,(there is only so many places he could hide,) and couldn't find him. To make a long story short,....we found a hole underneath our bathroom vanity that goes into the wall,....yup,....IN THE WALL. He goes in there now whenever we go in the bathroom to see him, and won't come out unless we put food out, or to go to the litterbox. He runs whenever we open the door. I'm clueless now as to how to build a relationship w/ him, so he trusts me. Just when I was thinking about getting involved again w/ helping kitties,.....here I am. I am at my wit's end trying to figure out what to do. Any ideas? Anything at all?