Absolutely agree with Cheerfulmarybrown.
I have had a rescue GSD and a first cross rescue myself, the pure bred would guard if given the ok from me, but unless I indicated I needed her to she'd be happy to stand down, iyswim. The crossbreed was never really into guarding, although that could have been down to her mix. She was totally aloof with other dogs, would say hello if they approached, but completely ignore them otherwise. Neither would be actively aggressive, even if the other dog showed signs of aggression, they would just offer some calming signals and move away.
The reason you don't see so many GSD's out playing with other dogs has nothing to do with aggression, but more to them tending towards choosing their humans' company over other dogs. They bond very strongly and often actively choose to stay with their person, rather than play with other dogs. Both mine were sociable to other dogs within our family group (our other dogs and those of my PILs and SIL) but really couldn't be bothered with any dogs outside of that group.
There are of course, always the cases where someone has picked the breed for it's reputation and others' reaction to it and fail to adequately train, socialise, exercise and stimulate their dog, resulting in them then ending up with an adult they do struggle with and can't let off the lead etc, but in my experience, they are far less popular as a breed than they were and for the most part this type of thing has moved onto other breeds who are now getting similar undeserved reputations, when in actual fact it's the owners, not the dogs that are at fault.
First thing you need is a full check-up at the vet, then a referral to a qualified, completely positive behaviourist. Try The APBC for a behaviourist that your vet can refer you to - and some insurers will cover at least some of the costs for and The IMDT for a totally force-free trainer in your area.
Also, if you are in their area - Surrey (I'm not much to my chagrin, because I would seriously love to take one of my dogs there) try Dog Communication. I know of several people that have been there and had great results with the help of their teaching dogs and carefully selected classes.
Absolutely avoid anyone that starts banging on about packs or dominance or suggests check/choke chains or any other punitive device to 'increase control'. You lad needs to be gently given his confidence back and that will only happen through steady, effective, force-free training and re-socialisation.