Then the first suggestion is have an eval done. Determine what you are dealing with. Could be anything from over excitement to fear aggression. Without seeing the dog in action, his body language, there isn't a way to give good advice on what to do.
I will make one suggestion however. Things like the soccer games, you seem to be able to predict when your dog will act out. Leave before he reaches that point. Also, keep the leash loose meaning no tension on it.
For the eval, locate a trainer experienced in aggression. Verify the experience and verify they are experienced in large working breeds - even better if GSD experience. You don't want the yank and crank variety, what you are looking for is a balanced approach.