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 socialization 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.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC