MarnaLou,
You are wise to keep him away from the dog park for awhile, until you find an answer for his aggressive behavior. My dog was recently attacked by another dog and both dogs were even on leashes, but the owner of the aggressive dog lost control of her dog. She said that her dog had never done that before and I believed her because she was really, really broken up about the whole thing. The vet referred to the bites on my dog as definite attack bites and not just warning bites. I was not angry with the owner or the dog because they are animals and unpredictable, but it has made me extremely cautious. I wish you the best of luck with your dog as a fellow owner of a "fur-child".
Julie