We adopted what we believe is as pure GSD from our animal shelter in February. We have no history on him as he was transferred from a shelter in a neighboring state. He was pretty high strung when we got him, didn't know his name, and very bad behaved. He's gotten better with some stuff, but we have a new and not good issue with him. He appears to have very bad aggression to any Male that comes over to our house that is not a part of our family. He tried to bite a friend of my daughter's a couple of weeks ago, and just today my adult sons are here visiting, and I made sure to try to introduce them and had Klause by his collar. He was being okay, and out of the clear blue he tried to bite my son. I immediately pulled him back and down, and he seemed to get the message, and we tried again after awhile and he tried to bite my other son. I then took him and put him in his kennel. He never growled or barked at either of them, just tried to bite. I feel like I can't trust him now. Is there any way to get him to not bite and be aggressive to men? He does not have this issue with women.