My toy poodle has huge aggression issues towards big dogs. She is all of four pounds and it started last year when we took her to an apple farm and while holding her ( I dont let her out of my arms in public places) a very large lab jumped up at her and snipped at her, I quickly pulled her away and we left. He didnt bite her thank god. Since then she has issues with big dogs, all of them, even ones she see's while we are in the car etc. She growls, shows her teeth and barks obsessivly untill the dog is out of site. I do not take her anywhere where there are big dogs if can help it. If we are at park (she is always on a leash)and see one coming we pick her up and go the other direction. She gets so stress out.
I agree with the others, your pup is too small to be running around off a leash in a dog park that has big dogs. Just the big dog playfully running around could cause harm to your dog and if your dog is aggressive even more problems could arise. Some one could get seriously hurt.
I also agree that you should have him checked out by a vet just to rule out the possibilty of health problems. Is he neutered? I know that some males will have agression issues when not fixed.