Aggressive toward puppies and elderly dogs

I rescued this little guy last October and have been pretty diligent in getting him to the dog park a few times a week. I noticed earlier this year that he was becoming aggressive toward puppies and elderly dogs. It has progressively gotten worse to where I have to leave the dog park when anyone brings their puppy into the park. I have tried more training (at home), focus exercises (which work great when there are no distractions...) and unfortunately when he focuses in on a puppy or elderly dog, there is just no stopping him. He attacks and has broken a leash to get at the other dog. Please help!!