Recall with a dog obsessed GSD!

I understand that this might sound sarcastic or rude, but I promise I mean this genuinely! Is there anywhere you can train with him where there aren't loose dogs? This is a super normal thing for an adolescent dog, but it's going to be incredibly difficult to work through this issue if he's constantly being (unintentionally) rewarded by play and interaction with loose dogs. And, as you've experienced, the risk of another dog being actively aggressive towards him is very real, and possibly physically dangerous - next time it might not be a small dog. He's still at a very impressionable age and negative encounters with other dogs could impact his behavior around them for life, so if you can at all find a way to avoid areas with roaming dogs, that'll be a huge improvement for multiple reasons.

DaySleeper is right. Generally dog training progresses when you can:

Teach the behavior in a quiet area with no distractions, such as in a room of your house without other pets or people around.

Once the behavior is solid as above, you move to a slightly more challenging environment such as a fenced yard when it's quiet all around.

Then you gradually increase the challenge of distraction little by little, either the nature of the distraction or the distance from it.

I know this must be particularly difficult for city dwellers once you get past the in house stage and must require real dedication. I'm fortunate to be a country girl, but even so I live next to a right-of-way dedicated as a horse trail which people also walk along with their dogs, so I have to keep an eye out and only train inexperienced dogs when no one's in sight. I also have a lot of rabbits that live in the bushes around my house and have take those into account. Everyone has to work around their own challenges.

Like Daysleepers said, try to find a place where there aren't other dogs about, or where leash laws are enforced, at least. I remember right between 7 months to one year old recall was iffy at best. It's a part of raising a puppy, at that "teenage" age they forget everything they learned and don't find you as the center of their world anymore! Of course keep the dog on a long line as you're doing, but it also helped me to just not do so much recall work, and make any I did stupid easy for him. It helped me to remember that his baby brain (or lack thereof) probably just didn't have the capacity to process both the "interesting thing" and my command.

When I was pretty sure he probably wasn't going to respond to the recall, just don't. Reel him in instead. Anything remotely interesting I just didn't call him. If I needed him close, I reeled him in, no recall. If he ended up coming on his own I rewarded. Any recall I did do he was less than 20 ft from me, basically just trotting along, no sniffing. I threw an enormous party and fed him hotdogs. It probably looked so dumb, haha!

Only around 10 months to 1 year old did I start upping the difficulty again for him. He had started to demonstrate his brain was reforming in his skull. Not doing so much recall didn't poison the word for him or give him many opportunities to blow it off.