
Maybe there's something on the way to the park that's upsetting him - perhaps a home with a dog that goes bananas when you walk past, or a particular smell - maybe you're passing through another male dog's patch and it's making your guy anxious. Has anything happened to him on the way to the park in the past that upset him?
On another line of thought, perhaps he associates the lead with something bad (I'm assuming he's off-lead at the park) - corrections from you, or an incident where it got tugged by accident or caught on something, so now he associates it with unpleasantness.