How long have you had him? For the first couple weeks, you need to more or less just let him adjust. Think about how his world has been turned upside down, even if it is for better. Do you have any experience with GSDs? They can be very reserved with strangers for a while, especially if he has been through anything traumatic. Do you know anything about his background?
Really, I wouldn't expect a whole lot of normal dog activity from him for a while, let him adjust and figure out his new situation before you make too many demands on him.