Both of mine, male and female, mark. They are really funny about trying to one-up each other. On walkies, if they are on-leash, they are on-duty, so less marking and sniffing, but off-leash (such as the dog park) they are free to read and piddle to their hearts' content. We call that "going to school", it's where they get (and give) their "education" about what's going on in the doggie world.
As for marking other dogs, your dog is a puppy still, so when he is introduced to other dogs at the park, if he's on-leash a gentle "not here, baby" and then show him where to mark within a step or two may help. I know my girl has been peed on because she is in a hurry to "see" what my male has to say and will get right in the middle of it. You don't specify if your dog is marking every dog, just certain ones, or if he's unaware the dog is there and is aiming for another spot. Is this a rare occurrence (has happened a few times but doesn't happen every time you go to the park) or does this happen every time you take him and you see he is aiming at a dog and not a clump of grass? Is the other dog marking a spot at the same time? If he is specifically marking dogs and you can't train him not to mark other dogs, then I'd stop taking him to the dog park altogether.
You have some good advice above, mostly that you have a dog and need to get into the dog's head and try to figure out what he's doing from a dog's point of view. Only after you understand his behavior can you redirect it. I've found that dogs, like children, respond best when I simply redirect them to what they CAN do as opposed to telling them what they can't do.