This is coming from a trainers point of view. I can't say for sure what is happening in Zig's case, but I can discuss a little about canine behavior. There are certain ways that dogs greet each other or expect to be greeted. If your dog if not giving the appropriate body language to the other dog, the other dog could be mis-interpreting his intent. If he is just standing there but is showing dominant dog signals (e.g., ears very forward, standing stiffly tall, tail nub held high), this could lead to the other dog believing he is challenging them rather than just standing there. I am just using this as an example. This may not even be Zig's intent but if he has not learned appropriate "dog language" this can be ongoing. Auntthelm is right about the throwing of paws over the shoulders or backs. This is seen as a dominating gesture and can lead to conflict. Honestly, to know for sure, it would help to talk to a very good trainer or like Auntthelm said, maybe finding a really good doggy daycare center who understands canine behavior who can help see why this happens. There are so many factors that can be at play here. Where is this occuring? At the park, at a friends house, walking down the street? Are both dogs on leashes? Only 1 on a leash? How well do you know the other dogs? How often has your boy had the opportunity to play and be with other dogs? How old are the dogs? All of these questions matter. The other dogs may sense that Zig is more submissive and they are asserting their position.
There is more to this than probably meets the eye. If it is happening with almost every dog he meets, then chances are you will need a professional to help determine what is happening and why. I don't know if this helps, but I hope Zig can eventually meet and play with other dogs as he so wants to do.