adeolescent male aggression - Page 1

My 11 mo male is generally very relaxed, social with kids, dogs, strangers, and i have spent a lot of time socializing him. With hie usual friends he is excellent, even tempered, not fearful. He is relaxed with visitors to the home. He is generally very calm and obedient on lead and off. But, in the last month, he has started to to be overly protective of the family, growling at strangers or lunging, especially at night, and in confined spaces. How can I best handle this? He is a working dog, black/red of slovakian/german lines. He is a pet, and neutered since 61/2 months.