Help, Male Mini Aussie Aggressive Toward People And Other Dogs

I have a 5 year old male, neutered Mini Aussie, who has been becoming more and more aggressive toward dogs and people he doesn't know. I am his second owner and the first owner told me that he was attacked multiple times by another dog when he was at a local dog park and for some reason hates males more than females but either way doesn't like either.

It took him months to get used to my brother and dad, but now he is totally cool with them. When I had a roommate, it took him 3 months for him to allow her to pet him without him growling. If someone comes into the house or around the yard he runs to the person barking and lunging; he doesn't bite them... just scares the people half to death, then he calms down enough a few seconds later and will listen to my come command.

He also tries to lunge at dogs when we are on walks; a few times he has gotten lose and he chases, barking and snapping at the dog till it submits and rolls over. Then he will leave it alone.

He is really well behaved inside my house and outside as long as no other dogs or people are around that he isn't used to (ie. everyone except my close family or our other family dog). I am trying to teach him everyday something new to keep him engaged and go on walks and play fetch in the yard off leash, but I can’t trust him if he keeps doing this.

I have tried to talk to trainers about socialization and they say I am doing everything correct when I correct him on the leash, but there has been no change that I can tell. Please, if anyone has any ideas I would greatly appreciate it. I am a college student and this is my first dog and the love of my life, but I can’t have him scaring my neighbors, attacking local dogs or heaven forbid a child that happened to ride by on a bike. I don’t want him to do anything that will force me to put him down. So please, please, if you have any suggestions let me know.

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