He obviously was very bonded to his previous owner, and now his entire world has been completely shaken up. Are you giving him the same diet that he was on at his previous home? Is he in the same cage, or is it a new one? Same toys, or all new? Do you see any food on the bottom of his cage? He could be taking some bites when you're not looking. They're not dainty eaters, and a lot ends up on the floor.
So many changes. You're right. It's going to take time. You're new and scary to him. Softly talking to him and sitting in a chair set just outside his comfort zone will be good. As you see him relax, you can scoot your chair closer. Play music for him.
He'll get there. Patience and time is what he desperately needs right now.