system
Given the following post, respond as if you're commenting on Reddit:
human
[Captain America: CW] A question about Antman

Not particularly spoilery, but I figured I'd leave the question out of the title: how did they convince Antman to fight? As I recall, his main motivation was to be able to see his kid on his terms and to be seen as a hero. 

The Sokovia accords would have either stripped him of his hero status, put him in jail... or given him legitimacy. Fighting against them would only make him seem like more of a criminal, and while he's not an immoral man, having a fight with representatives of 117 nations in a German airport doesn't seem like it would further his goals.
gpt
Most of the comments here are missing why Cap was at the airport in the first place. Cap was putting together a team of people, not under the accords, to help take down Zemo and the winter soldiers. 

They weren't expecting Stark to show up at the airport. They hear announcements about the airport being evacuated and Cap says, "Stark." to which Ant-man responds "Stark?!"

He isn't there to fight the accords, he is there to save the world. Iron Man and his team become an obstacle to Team Cap and Ant-man supports their mission.