imo, this is a tough one, it is likely that he has learned that humping is how to interact with other dogs... interrupting and re-directing him when he does hump will help, but since humping is a self-reinforcing behavior, i think you will have to spend some time teaching him some alternative behaviors... basically i would have gobs of treats, have him meet another dog, and reward him for all interactions that aren't humping... i would also work on building some value for a specific toy, something that he only gets to play with, with you, with rules...
I found this link both useful and hilarious...
if you make the toy special, then it is more likely to work when you try to re-direct him...
as far as the dog park, i have mixed feelings about them... you might try forming some smaller doggy play groups and socializing him with other dogs during these, smaller groups, you can try off leash hikes and things (if he has a solid recall) are great ways for dogs to "hang out" without having the pressure to socialize with one another that a dog park can cause...
anyway, hope that helps...