If I can suggest you something....AVOID DOG PARKS.
We have made this error with our dog. We wanted him to be socialized a lot (because we've been told GSD need a really good socialization otherwise they were agressive toward other dogs).
As soon as we could, we brought him to the dog park, maybe around 3 to 7 times a week.....at first, everything was great! Phenix had friends (especially Pitts, Bulldogs and Rottweilers).
Than, one day, what should happen, happen and he got seriously attacked by an other dog.
The thing is you cannot know each other dog and owner in dog parks...some of them are very good socialized and sweet, but other could do damages to your puppy.
Since this event, Phenix is TOTALLY scared of other dogs....we are working so hard to socialized him again. I will NEVER go back in a dog park for sure....I really regret that we brought him that much to the dog park.
Instead, you should bring him to puppy classes. This is more fun, he will learn and he will be socialized with a good supervision.