Anger Management

I have a German Shepherd, who is two years old now and he loves people but is terrified of dogs. Lately he has been showing aggression to children. He’s nipping, snarling and even lunging at kids who try to pet or play with him. He’s never shown this kind of aggression before and I don’t know what to do. Is it because he isn’t fixed? I need help. If this problem isn’t foxed soon I could get complaints or he could hurt a kid: my worst fear. Please give me your opinions.