Miri, you're going to get a lot of replies to your post, some of them probably rude, fussing at you for how you handled that situation. However, you do need some advice and help, so please read it all carefully, don't get your feelings hurt, and do some research on this site for help on how to train your "reactive dog" not to be reactive.
Basically, you overloaded your dog with too much stimulus too early in your relationship. Your dog needs you to be a leader he is close to and he trusts, so he can rely upon you to be his protector and he can trust you to take him only into safe situations. If he doesn't have this relationship to you he has to then decide on his own how to react to strange and threatening situations. In this case he felt threatened by Pash and the other dogs in the park and reacted in the way dogs naturally react--he became aggressive and unfriendly.
You need to read about the "two-week shutdown" and learn the principle behind it. Basically, you need to devote several weeks to establishing a trusting bond with your dog, just you and him.THEN and only then should you begin introducing him to small, secure, fun outings--short walks, meeting only one stranger or one friendly dog at a time, etc. As he gets used to these stimuli, and becomes more trusting of you, he will relax and begin to act more calmly around them.
Your concern is admirable, but your post seems to say that you are very confused about how to raise and train a dog. That is OK--we all start off that way--but you MUST educate yourself with lots of study on how your dog thinks and reacts and train yourself to be an appropriate owner.
READ, READ, READ!
Liesl, b. 1/1/11