[Help] Dog's showing aggression to other dogs at 3 years old - new behaviour

I have a 3-year old Chihuahua (full sized, long haired). He weighs about 13 pounds right now (and is on a satiety diet to lose weight - he should be around 10 - 11 pounds), and stands about 1' tall. He's a small dog, of course, but not as small as typical chihuahuas.I adopted him when he was 1 from a family that, though there's no concrete proof, we believe used to hit him to get him to behave. There are indicators there, and two different vets have confirmed that it's a realistic possibility due to his inherent distrust in humans. He's terrified of men, and will not go near them; women he's scared of too, but warms up to rather quickly (typically within 10 minutes of meeting them in a 1:1 setting). He's never shown aggression towards humans before.After I got him, I noticed that he would be a bit growly/snippy towards larger dogs. The vet and the trainer (we did puppy socialization training) both said that this was to be expected from a non-neutered small dog trying to assert his dominance. He never bit but he would snap until they learned he was "in charge", then he was fine. He's typically ok with dogs his size or smaller - often either ignoring them or hiding if he's scared (in which case we diffuse the potential situation by leaving the area).We got him neutered in December. In the past two months it's seemed like he's getting more aggressive - period. He now growls (butt and tail up and wagging, front paws down like he wants to be chased) at almost all dogs. He snaps more frequently and aggressively towards big dogs (i.e. yesterday a neighbour stopped at my door on the way into the building with her medium-sized dog. Mine got out of the front door and immediately got into the growling-play-like stance and snapped at her seconds later. She wasn't looking at him or doing anything to indicate that he should be threatened). The other thing he does is chase my neighbours down the hall in the same low stance; however, as soon as they turn towards him, he runs away. I'm not sure if he's playing or play-attacking but I don't want him to end up biting anyone.Has anyone seen this? He gets exercise and food, he's well cared for and no one hits him anymore. It's mostly been the past few weeks and I'm concerned about this new aggression. via /r/dogs http://ift.tt/2qOLZ9H