Go to an obedience class (which will HELP with socialization & BONDING) or have a private trainer come to your home, to help you learn how to train the dog to walk on lead. I am not sure
a) you have properly leash trained the dog and
b) are using the right equipment to prevent or discourage pulling.
We KNOW (from your post) you do not know how to teach the dog to heel on lead. Virtually no one does..... with out TRAINING. You have a large protective breed and in your case a dog who sadly lacks proper socialization. so a calmer quieter obedience class with puppies under some "slight" control and also on-lead, would be far better than a dog park.
"A calm dog is a better behaved dog, because it has no choice." GSD are a high energy breed coupled with HIGH intelligence; they do need a LOT of BOTH mental & physical exercise, so a walk is NOT the only way to provide such. (FAR FROM IT!)
In fact, if you can find a fenced, but empty field or even a cemetery (NOT POSTED as "off-limits" to dogs) that he can run inside of - he needs to some off-lead exercise. If he gets 30 to 60 minutes of hard running (at his OWN speed) before you attempt to walk him, it will go a LOT better. (I say this having owned a Weimaraner, who HAD to run for an HOUR before she ever entered an obedience class). If she did NOT run (for at least an hour) she looked like the "demon-possessed" girl in the EXORCIST movie - with her head spinning around and in her case, NONE of her feet ever seemed to be on the ground.
If he will retrieve, you should do that was well. If he has any "dog friends" (or develops any, in his obedience class) he can go on a play date (to their house, with you in tow) or have them & their owners come play at YOUR yard, as another outlet. I would also suggest you look into: tracking, nose work, herding & agility classes, as well.