Keep him within 20 feet or so at all times until you assess the situation. Do NOT automatically reach for your leash and say that you'll just keep the dog on leash while in the park. This seems like natural solution and while leashes are not forbidden in an off leash area, they are strongly discouraged because it changes the communication dynamics of dogs. If you insist on using a leash, please read the Calming Signals book mentioned above as it will allow you to see communication as it develops in the park. It also prevents a dog from avoiding an encounter with a more assertive dog if that would be natural inclination.
Dogs are dogs.
This sounds obvious, but some people have so personified their dogs that they forget natural dog behavior is likely to come up when dogs are around other dogs. It is Natural and Even Positive. In fact, they are play fighting. The key is to not panic and correct your dog for entering into this natural play. The dog will simply learn that it gets in trouble if it tries to communicate naturally with other dogs.