Well, the first thing would be to get him our of there! lol.
After that, you have to understand that a male dog is prone to dominance. So if there are two, male dominant dogs in the same area, they're going to fight and see who wins.
Of course, it could also mean that he's not feeling well. You should watch for symptoms and if he's acting strangely in other situations too, then maybe a visit to the vet is in order. (When people aren't feel well, they get grouchy, too.)
You need to watch his behavior, in general. If he's only acting badly at the park, then it's possible that there's something there that's troubling him. Take him for a walk around the block instead, and see how he acts then...