my neigbour bent down to greet him and my dog went for him. I had him on a lead
This situation of someone towering over a dog can be very intimidating to some dogs. If you imagine the scenario from Zackys point of view, he's leashed so knows he can't escape and has someone five times his height is towering over him, very intimidating. Although you know your neighbour was ok and not going to harm Zacky, Zacky didn't share this belief and was telling your neighbour to back off. Growling is a dogs way of communicating, we have speach, we can say "sorry please go away I don't like you" but Zacky didn't have this so responds the only way he can
My dog is vary wary of strokes too, he is fine with strangers until they tower over him, he hates it absolutely hates it. Like Zacky Billy was about 18-20 months when he first showed any signs of agression in this manner. I do think that the fact that he was fine with your neighbour later on shows that it was the scenario rather than the individual that was the problem.
I do think you need to take this as a sign that there may be scenarios that Zacky isn' comfortable with and try and build on this. I do not allow ANYONE to stroke billy now, instead I give them treats which they give to Billy. My aim is that Billy will eventually realsie that all humans are good, whether it will be enough for me to allow people to stroke him is something I'll have to see.
I'd b wary around children, make sure they know how to behave around Zacky - better to be safe than sorry.
