I wish we could have watched the cousins run together, I'm sure that would have been a lovely sight. But with all the lure troubles and whatever was going on across the field, I guess we'll have to try again another time. lol
My advice is to leave the park the moment you see a dog you believe may trigger him. And make sure you see it first!
To change your dog's reaction, you need to begin a desensitization and counter-conditioning program. Which means that you cannot expose him to his "scary thing" to the intensity such that he begins to growl. Once he's tipped into an emotional behavior, you've missed the boat on your opportunity to change his internal reaction.
If he's growling at say, 20 feet, you can probably start to notice other warning signals before hand, such as a freeze or hard eye. Even that's too late. You have to get to him before he starts to tip - while he's still comfortable. Which means working in a controlled environment. That is with dogs on leashes.
A good trainer can create a training program for you and work with you on this, but repeatedly exposing him to large, dark dogs and waiting until he's already growling to get outta dodge is reinforcing his fear. And the recent scuffle really reinforced it. I'd say you really need to up your situational awareness or start skipping the dog park. Every time he has an unpleasant experience with a large, dark dog just tells him he was right to be uncomfortable, even if the other dog does nothing but appear.
