"He was not well socialized or exercised during this period"
You need to gain your dog's trust again. He stayed with your grandmother, obviously something happened to him there, plus you have a new Dutch Shepherd. He needs some extra, special attention. Try a group obedience class, like a refresher course. You'll be spending one on one with him plus you can positively reinforce his good characteristics and an objective eye from a trainer may see something you are missing and your dog will get used to being around all types of people in a controlled environment.