I wouldn't take the chance with a 12 week old puppy.
At twelve weeks a puppy is liable to get injured running and playing with large dogs. A pup this age's immune system is immature and the vaccinations are neither complete nor do they cover everything that is running rampant where populations of dogs congregate. And, pup's this age are in a critical learning stage, where the learning tends to be permanent -- good socialization experiences are very good because the dog can draw on these experiences when he confronts other new or similar things. However, bad socialization experiences are very bad, because a dog can actually draw on this experience when he sees other large dogs.
In life there are few absolutes. One is that we cannot control all the other people around us. We can only control our own self. Which is to say, that there are jerks, and we cannot be certain that they will not bring their puppy-eating-crazy-lab mix to the dog park, so they can have a romantic dinner outside. Instead, we have to do what we can to protect our critters. In this case, I would say take your puppy out of the dog park at least until all the vaccinations are complete -- he will be a little older than, and the highly-effective socialization period ends about the that time, 16 weeks give or take.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC