Scott and Tony meet in the superforms for the first time. When we last saw him, Iron Man did this…
With literally every ounce of power in his suit.
In the aftermath, Tony Stark is trapped inside his armor–which used up all its power to knock out Hulk–and he’s running out of air.
So, it’s Ant-Man to the rescue, going inside the suit to disable its safety features and let more oxygen in.