There are certainly some dogs who become pariahs. Certain dogs just don't communicate correctly, which can predispose them to being attacked. Very energetic, young adult dogs come to mind...

However, it also sounds like your dog has some fear. Because he's been in so many attacks, it sounds like he has adapted to being overly cautious and aroused around other dogs. So, when he sees strange dogs on a walk, he'll go into a "uh-oh-what-is-that" state of mind. Then, when he sees the dog is producing friendly signals, he himself will relax. I see this with a lot of dogs who don't know how to communicate well. Dog A will see Dog B. Dog A is so concerned/aroused that he'll stand tall and rigid with staring eyes. But when Dog B gives a friendly signal, Dog A will immediately relax and give a friendly greeting.

When he jumps on top of other dogs, does he put his paws on their backs? If so, it sounds like your dog is trying to display dominance and *that* is what's getting him attacked. I strongly suggest hiring a behaviorist to come watch your dog interact with other dogs. They can help you decipher how your dog is communicating incorrectly and give you insight as to why. It's money well spent, I promise!