Was the puppy with you?
Dog aggression at a dog park is easy, just don't go to the dog park anymore. Not all dogs, not all GSDs, are dog park material. And if that was the only problem you had, I would say, fine. Just don't take the dog to a dog park, exercise him with walks, runs, hikes, etc. Done.
What a lot of dog owners don't understand, is that dogs can go for the rest of their lives without canine companionship. They do not need to meet the friend's or family member's puppy to have a good time. Leave the dog home when your friend or family member has a dog. Easy. Done.
What you need for your dog to do, is to not turn into a maniac if it sees a dog on a walk, and not bite anyone when he is waiting his turn at the vet, and people are walking dogs past him. Of course you could use a muzzle at the vet. But it would be nicer if you didn't have to.
But it sounds like you need to go beyond basic socializatioour dog if you min and get his behavior under control through regular training.
Regular classes will help you train your dog to walk nicely on a lead, to use the proper tools if you choose to go that route, if that would be safest, and will work with the dog around other dogs, not so that your dog can run and play with them, but so that he will be able to be under control in the presence of other dogs.
That's excellent advise but I'm gonna add to that!
First head halters are crap, you run the risk of injuring your dog if you misuse it. If your dog won't heal "your" doing something wrong, maybe a trainer is the answer but not new crap.
You don't trust your dog, why put your friends puppy at rest as well as your friendship? You have one dog with "issues" and your adding a puppy?
I would in no way trust your dog around a puppy myself, I would treat your new dog like a new dog. Crate, drag leash, and keep him out of striking range of people and other dogs,better still find a good trainer,not Petco for a trainer.
Leerburg | Introducing a New Dog into a Home with Other Dogs
Sorry if I sound a little over the top but we have lots of "my dog bit..." or "my dog attacked another dog threads.
You accepted responsibility and reached out!
If I had done that with my guys and my rescue GSD at the first sign of trouble..I would not have needed to learned the "correct way to break up a dog fight!"
And I had big dogs! So I learned all this stuff the hard way and I had well behaved dogs in all the situations you described but the GSD made three!
New Ball game, We all have stuff to learn! Good luck.