Some GSDs do not like the hyper-crazy thing. If you have two young males or two young females interacting, it could be hormone thing. Unless they are both fixed.
Some GSDs are friendly, social, dog-park-types, and others are not. Your dog is just entering young adulthood so his overall temperament may be not the kind of dog that wants to run about and frolic with other dogs.
Today I had my bitch with me at training class. It is an advanced class held on Thursday when I am usually working so I dropped in today as I was not. My girl will be nine in one month. There was a small dog and a couple of young adult goldens. Babsy is super-trained and good on leash or off leash. But when the Golden broke her stay, and came over to check out Babs, I took Babs' collar and prevented a face-to-face connection, because while everything probably would have been fine, I did not want the young golden to go home with a gash in her face if it wasn't. I have never allowed Babs to frolic with other adult dogs, and I don't think that we need start that now.
It may be that way with your dog. Perfectly fine, great with people and dogs, so long as everyone is controlled and no one is running around in his face.
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