There are vet checks and vet checks...Did he have a proper thyroid panel? Sometimes it can cause some mood swings/changes.
And yes you can make your dog respond....if you are anxious/nervous your dog will often react.
We teach our dogs a greeting habit.....We say "who's that" when we see a dog and get the dogs to focus on us...often in a sit and treat. Focus is clicked, but that is optional. Everytime we meet a new dogs I ask "who's that" and because our dogs have been conditioned to look at us (focus).they look at the new dog and straight at us. this is a break...it gives time for adjustment. We will often ask our dogs to sit/drop, even from a distance. This way we take back control and we can often re-call easier, if we are not sure of the other dog. We also teach "say hello", which is when we will approach the new dog side on and allow a greeting with suitable dogs. But , because we have done so much dog pass dog training and "who's that". Our dogs always seem to look at us first, which gives the pause often required to get the control.
In my eyes if my dog was likely to go for any dog, I would not allow that dog off leash until I was able to do these exercises on leash and after that in a more controlled environment, maybe first even on long leash and then with familiar dogs.
I still have to watch Annabelle, always, because she used to rush dogs....i don't think in her case it is dominance, it is just bad manners and attitude. Now she looks at me first...i will tell her sit anyway when i see a dog I or her don't know...if I am worried, I do drop. We do the same with Lukey, because he is a 5 year full male.......
I also like to proof my wait, to get control on the dogs. I am never sure wether some movements dogs make are dominant, I often think it is fear...I like seeing their faces and check for their yawning, eye flick, licking lips or looking away ( mouth and ears)....very few dominant dogs attack, they are too sure of themselves. it is usually the worried dog that gets the first bite in quickly. Annabelle was classified as a dominant aggressive by some trainers, but she is a fear aggression, who got confidence.
Try getting the book/video by Turid Rugaas. I found her books enlightning
And here is an Ian Dunbar quote
Growly dogs
What if you have a lot of difficulty stopping the dog from growling? Are we dealing with a dreaded dominant dog? An aggressive cur? An alpha leader of the pack — planning to take over the family today and tomorrow, the world? Most likely not. Characteristically, growly and blustery dogs are middle-ranking males, who have limited experience and are insecure of their social standing and so, usually resort to bluff and protracted threats. (Not too dissimilar from people really.) Often the dog may growl incessantly to add major emphasis to a minor point. Most overtly aggressive dogs are all bark and no bite. "If you really have it, you don't need to flaunt it." Indeed, a true top dog is a rather cool and relaxed customer, who very rarely resorts to lengthy threats, let alone prolonged blustery bluffs — he doesn’t need to. Instead the threat is subtle and the follow up is immediate, short, and sharp."
