Hi everyone, I have a male GSP intact, he will be 2 years old at the end of January. Recently we have been experiencing a serious issue with him and aggression towards strangers and I am looking for others advice on ways to fix this behaviour. 

He is not aggressive to all strangers, on leashed walks we can pass strangers without any problems. He doesn't have problems with having strangers inside the house during parties or get togethers or when friends come to visit. He will bark when the door rings but settles down after he gets a sniff. He will however act up if these strangers get their faces too close to his (bending over towards his face etc)...

The behaviour we are concerned about is during our nature walks, we let him run off leash. It's usually isolated forest area but occasionally we will have passersby, and this usually is when he begins to act up. He will start to growl and bark at them from a distance and the last time we took him out, he lunged at a non threatening stranger. This is the first time he has ever done this, and we are trying to figure out where this behaviour is coming from since he has never done this before... When he acts up, we correct him, tell him NO, BAD DOG and make him do a sit/stay for a few mins. 

We are quite upset at this, and wondering what can be done to help fix this behaviour before it gets any worse. We will obviously have to keep him leashed on these walks for now until he is better. Would getting him neutered help? Any advice is appreciated

Thanks