Couple things about a recall.
A dog should never be off leash if his recall isnít reliable. I know, but itís a fact. I have acreage and right now, two of my dogs are always on long lines. Itís inconvenient as heck, but if they donít come when I call them, I canít enforce the recall cue. If they run the other way, Iíve trained them ďcomeĒ means ďignore my human.Ē Itís basic training.
Recall is the most important thing I train my dogs. I start training it the day I get them, whether puppy or adult. Itís what will keep a dog alive. We canít undermine our own critically important training.
A longline at an offleash dog park sounds like a recipe for injuries, human and canine. To be blunt, donít do it. Train your dog elsewhere.
GSDs play in a way that most breeds donít. As one of my old trainers used to say, ďthe body slamming, the nipping and that big paw on shoulders, it pisses other dogs off.Ē Some offleash parks are fine. Some arenít. But our dogs donít play like most breeds. Some dogs adapt to it. Many donít. In my GSD pupís puppy class, he and another GSD play happily together. They tend to freak other pups out. And the way the labs play, with no regard for personal space, that kind of bothers them. So I find itís best to find other shepherds or dogs that live with/play with GSDs to socialize with.
As for recalls, throughout your day, all day long, you should be practicing your recall. Just telling him to wait in his crate then calling him across the room, calling him to dinner, calling him to bedtime, etc, make recalls part of your routine. Reinforce those recalls with praise, petting, play and treats.
The dog should come instantly & enthusiastically to you, front and center, with no fly-bys. If he consistently isnít doing so, you may have burned your recall word (let him ignore it so often that it means nothing) or you may have poisoned the cue (called him but you were angry or then did something he didnít like). Pick another word. If you use ďcome,Ē you might use ďhereĒ or ďfront.Ē
If you have kids, a partner or roommate that calls him without follow up, donít tell them the new word. Burned and poisoned cues are more deleterious to training than many people realize.
If you call the dog once and he doesn't come, go get him. Take his collar (using a tab is even better) Donít touch him. Donít make a big fuss (attention and touch are reinforcing), just get him. Obviously, you donít want to have to chase him (being chased is fun!) thatís why he needs to be on a leash or long line until his recall is reliable.
Heíll quickly learn that ignoring you doesnít do him any good. Complying with a cue gets happy cheering, play and treats. Ignoring gets a boring escort to where you wanted him to go.
Meanwhile, keep working actively on your recalls. I donít use correction (other than an ďehhhĒ or a soft ďnuh-uhĒ as a nonresponse marker). The instructions above are generally good otherwise.
One more note, if you have a dog that has excellent handling skills, particularly if heíll drop to a down instantly for you, work on that. Increase your distance. Get your dog trained so that he will down from anywhere. Work on a rock solid down and a rock solid wait.
I have a beagle and a Samoyed/GSD mix (Samoyeds and beagles are both known as runners). Both of them I can call their name and hold up my hand, they stop and wait. If I drop my hand, that means down, right there regardless where they are .
If I squat down, they run to me. Or I go get them.
With the Samoyed mix, the deer &elk on our property compelled her to chase. At first, I couldnít proof a rock solid recall as I was working against her genetic instincts. But I could get her to stop moving after the deer. Once she stopped, I had her attention to call her to me.
I was able to convert that into an actual recall over the course of about a 6 months.
There are times when we want a dog to just stop anyhow, like if he has run across a busy street. We donít want him running back to us.
Anyhow, thatís an approach for a dog thatís super focused and needs a break in focus before being recalled. Itís easier for a dog to stop and wait then recall. You just have to hold the dog in the wait. If he dashes off when you approach, he must be on a leash.
But I donít see that that particular dog park is the right venue for him, at least not in the near future. Remember, positive training is fun and works his brain. You probably donít need the dog park.