dnd 3.5e – If the goal of Staggering Strike has already started an action, can he do something else?

With Staggering Strike, if an enemy moves and causes an attack of opportunity, hit him, and fails salvation, he becomes stepped.

Staggered

A stepped character can perform an individual movement action or a standard action in each round (but not both, nor can he perform full action actions).

A movement action has already begun (that was the trigger). Is he now unable to do anything else in his turn? Except maybe a quick action.