Although the other respondant (xemo) is right about why dogs hump (it is a natural thing and a show of dominance), I would not encourage prozac unless it is absolutely necessary. (I dont belive in over-medicating dogs).
Just carry a spray bottle with water with you to the park. When your dog starts to hump another dog, just spray your dog with a little water. He will be suprised and annoyed and stop humping. When he goes back to do it again, spray him again. This way he associates humping with being sprayed and he wont do it. The second he stops humping (after being sprayed), say "good stop" and give him a treat. That will make him learn it even faster.
You also have to think about why he is humping in the first place. Did something happen (an attack by another dog) that makes him want to prove he is the boss of other dogs? Good luck!