First thing to realize is that the aggressiveness comes from your pup being a bit afraid of and intimidated by these larger dogs. So if you understand that, then you can realize that any punishments or negatives you add to his world for his behaviors, will only increase his discomfort and fear.
So what I would do, is to not put him in a situation where he has to deal with these bigger dogs. Don't let him loose at the dog park with them.
Instead, make sure your pup is getting to play with other dogs he likes, in a controlled environment, perhaps just one or two playmates at a time, and for short periods so no one gets over tired and grouchy.
This is a time in his life when you want him to have Positive experiences with other dogs, so he does not become fearful (or in his case, more fearful). So you want to just plan those social events. Quality over quantity is your motto!
And certainly don't scold the puppy when he gets fearful-aggressive. It will make his behaviors much worse. Just gently remove him from the situation, and plan to prevent it next time.