How can we get our dog out of the corner and unafraid of people?

We are fostering a black lab mix who was rescued from a hoarder with 60 dogs who constantly fought. He has severe social issues and is not housebroken. He will hide in the corner for a very very long time with out moving an inch. He is disinterested in anything we try to lure him out with and have tried just leaving him alone. Any tips on how to break these habits?