Aggressive GSD Puppy?

Hello,
I've owned two purebred GSDs before, both rescues. One was adopted at 1 1/2 yr and the other at 9 mo. They were awesome dogs, laid back, great with people, never any behavior problems. After our last fellow passed, we decided to try getting a puppy from a breeder. We met both parent dogs and brought a male pup home. He's been with us a month and is currently 5 months old. He'll be getting neutered next month. My concern is that he has acted aggressively with visitors at our house. He lunges, barks very aggressively, and I think he would bite if not restrained. He is unpredictable about it too. A friend of my son's was over and he was completely fine with him, and then the next time he visited, our pup acted like he was going to attack him. Same thing happened when my father-in-law visited. Fine with him first time and then vicious the second time. Any ideas why he'd act this way, and any advice? He's so different from our first two dogs.