Abused Dog Help?

I have adopted a beautiful Shetland Sheepdog. He is 8 months old and has had past history of being abused. I don't know how to make him trust me. He is very shy. He walks around the house with his tail in between his legs. He is skittish, any slight sound will make him jump. When I reach out to pet him he freezes and gets scared. Whenever I go up to him, he crawls in the corner and gets upset. It breaks my heart. When I call him he won't come to me. He is very smart, but most of the time he will hide under my bed or couch. One time he got in a small hole to get underneath my house. I couldn't get him out, until he got hungry and needed food. I don't know what to do. He won't trust me. I have worked with him, but he still holds on to the past.