Aggressive German Shepherd

Hi,
I need some advice and no one to ask.

I have a 5yo German Shepherd that can be dog aggressive but becomes socialized when he's around other dogs routinely. My problem is that when I am out with him for ON-leash walks and exercise, that other OFF-leash dogs frequently attack my dog. Old dogs, young dogs, male, female, big or little it doesn't matter. Walking down the sidewalk, in the park or wherever it happens all the time.

My understanding of this is behavior that the other dog(s) perceive my alpha dog as a threat and so they attack my dog first, thinking they are protecting their owner from a perceived threat. I always fend off these fights and run the other way but it causes me a lot of problems with the other owners and sometimes the police, and most of the time the other owner is astonished to see that their dog attacked mine.

I want to socialize my dog but I don't know what to do about other dogs attacking my dog when we are just minding our own business. What I do now is to stay away from other dogs but then that works against socializing my dog.

What should I do?

Thank you for any advice.

Regards,
JC