If you got him 9 months ago, and he was thought to be about two years old, he may just be maturing into his big-boy pants. GSDs are considered puppies up until two years old - they mature slowly. So it is just his big-boy-pants syndrome coming out.
Agree with the others. This behaviour is not likely to change, if anything, may get more serious. Once he has had a tussle with one dog, his system is flooded with fight hormones, and thus he may be looking for another outlet for his aggression - so just a vicious circle made worse and worse.
A lot of dogs are fine at dog parks while still puppies, but not so much once adult - agree with the others that you may have to stop the dog parks, find more isolated open spaces to exercise him (or just take him for a run, or a hike), and set up social dog-dates only with dogs that you know he gets along with.
BH, OB1, TR1, AD
Rottweiler/Hairy Dog mix?? Shelter rescue
Gryffon Vom Wildhaus