You need to look closely at his hackles.
If they are standing up over the length of his body, it means that your dog is trying to decide whether to react in `fight or flight' mode - usually flight, and its a sign that the dog is unsure.
Hackles up from the shoulder forward means he's going to attack.
Did you notice how his hair was standing up? It may be that this was just a new experience for him and he didn't know how to behave, hence the growling, too.
I would take him again, but not let him off the leash OR, even better, make sure he is muzzled until he starts to get used to being around other dogs. All dogs need to be socialised and avoiding the situation will just make it worse the next time you take him. Desensitising and socialising will improve his behaviour immeasurably.
A muzzle will show other people that you are taking responsibility, and you can also explain that he is not aggressive, but while he is new to the park, and you are teaching him, it's better to be safe than sorry.