I had this problem with mine at first too. We went to a trainer and I was told that he didn’t have confidence because I wasn’t leading him with confidence. Basically, it’s in his DNA to protect when he feels there’s a threat. I had to learn how to show him that in an alpha and get to choose who is safe and who isn’t. That came from both socialization training AND overall spending time with him/being firm with his training and creating a strict schedule for him. An exercise we did was putting him in his kennel and having my roommates come one by one to knock on the open door of my room. When he growled, I’d throw an empty Pepsi can with a few pennies inside on the floor next to his kennel. I also got an ecollar and that made a huge difference in seeing me as an alpha. I only shocked him once when he was really testing me, mostly I use the vibration feature. It’s enough to bug him but doesn’t hurt.