At the moment his mounting sounds as if it is still play behavior. However, I strongly suggest that if you don't get your dog neutered you stop taking him to the dog park. Remember Cesar says male and female dogs that are unable to fulfill their drives become frustrated. If you do not intend to show him in conformation and stand him at stud there is no reason to keep an intact dog.
As for the shyness, I'd continue what you are doing. Tell anyone who tries to approach the dog on the street that he is in training, etc. Make him sit and and relax while you chat or let the others pass. Use the collar to correct him when his body posture indicates he is about to move behind you or growl. You need to do the correction when his posture changes. If you wait for the growl you risk the correction being late.
Work on the "watch" command to get your dog's attention on you when you are walking, sitting or whatever. When his attention is on you watching for "what's next" he isn't paying attention to passing people and dogs.
I also recommend taking Cesar's Sessions if you haven't already. They give lots of help on body language and corrections, but you really need someone to who knows to observe you. No matter how experienced you are, your timing can get off.
You also need to deeply examine any feelings of shyness or timidness that you might have when a strange male approaches. I'm assuming, since he is shy around men that you are a woman. Often we be apprehensive around strange men and not even realize it. Remember, Cesar says that our dog is our mirror. The dog picks up on our feelings and takes control of the situation if he feels we are no longer pack leader.
If you aren't in obedience classes with him, that is a good controlled place to let your dog socialized. He will learn to obey you around other dogs and people on and off leash in a controlled environment. More important you will learn how to time corrections and praise and read your dog's body language. You can find qualified instructors through these websites:
http://www.akc.org/events/obedience/training_clubs/
http://www.canineprofessionals.com/default.aspx
Interview the instructor and ask to observe a class before committing. Also talk to people who attend or have in the past. If you aren't comfortable with an instructor, keep looking.
Whatever you decide,The fact that your dog acts fearful and sometimes growls can turn into fear biting. Remember, stay calm assertive and keep him calm submissive.
He shall be everything humanity strives to be: Honest, Loyal, Kind in Heart, and Strong in Spirit. Therefore, you shall call him Partner and Best Friend."
Author Unknown