His adrenaline was still coursing thru his body from this mornings incident. I would work on training with him away from that park for a few days, take him somewhere else that is fairly busy yet you can stay on the fringe of the activity.
When you do take him back I would expect he will be reactive/his age is playing into it and the experience will still be fresh in his mind.
This is the age when dog parks and GSD's don't really blend well. It was good while it lasted, but maybe time to find other options to exercise him?