Well, it's too early for your puppy to turn agressive. He is, according to photo, in the age of angelic being. At one year old he would be naughty and disobedient, and, expect to deal with his agression when he would be about two years old, it varies in dogs. Your dog will expose different attitudes during these phases to, not necessarily to one and the same, but to similar things. If you don't attend any classes, try to find a doggy park for puppies in your area, for a start of training him good manners it could be a good idea to satisfy his thirst for communication with his kind. From here you start to divide for him all dogs into two groups: 1) those whom he is allowed to greet; 2) those he musn't approach at all. At the puppy stage allow him to approach mainly only puppies. It doesn't matter what his doggy's reaction might be, he must listen to the known to him commands. Ask him to sit in every situation you see him excited. They calm down in sitting position. Repetition of this procedure, when he sits and waits for a dog or a person to continue walking towards him teaches him patience. Of course, your "OK" will work as a treat.
Ask him to lie down and watch another dog walker passing away. Let him bark, if he wants, but he musn't get up. If you are in the street - ask him to heel and turn round 360 degrees together with your dog, see, how well he would be at this. Teach him to make rounds around you if you see him too excited.
It is OK, if his attitude towards some dogs turns negative one day, this fact is not important as long as he is obedient to your commands. In the future he could be boiling with fury, still, walking glued to your left thigh, without any lead restraining his freedom; he likes or dislikes the stranger, it shouldn't be an important fact, he must sit if you told him, and wait for the next command. You cannot be hard with babies, such thing as discipline is alien for the young. But, if you persist in your task of teaching him as a forbearing parent - you can teach him to become a very pleasant dog.