I used to know a Mail Carrier who carried an airhorn in his bag to fend off dogs - he used to tell me that sometimes by the time you know whether or not the dog's aggressive, it's too late. It's loud and everyone will turn to look, but it may startle the dog enough to stop him. This person had tried a "dog whistle" (the silent kind people can't hear), but didn't find that much help. All the dogs will hear it and none of the people will, but it might be enough to startle him and make him stop long enough for you to get away. Some Mail Carriers carry pepper spray with them to fend off attacking dogs. This is extreme, but if nothing else works it might be worth considering - it's extremely uncomfortable, but it isn't particularly harmful. My Mail Carrier friend was very anti pepper spray (not because of dogs - he always had a fear of getting it on his hands and forgetting, then rubbing his eyes) - that's why he carried an airhorn.

There's a training product called "NoGo" that supposed to keep dogs from using your yard as their potty. If you can stand the smell of that (it's pretty noxious from a people standpoint), spraying that on a rag and waving it in front of the dog might slow him down. It probably wouldn't be effective when getting jumped on from behind.

Perhaps you could consider talking to some of the other visitors since you're not alone when it comes to this unwanted attention. I've wondered how situations like this are handled at dog parks; there's one being added to our community. We're anxiously awaiting the opening but I worry about situations like this.