You can find Steven at the very top of Meteor Falls. You'll need a Pokemon with Surf and Waterfall to ascend to the top of the waterfall near the entrance. Take the first ladder in the back room, follow the path to the northwest to another ladder, and then climb up the staircases to reach the highest point of Meteor Falls. Steven and an old woman will have a long conversation. Once that's done with, fly back to Rustboro.
Comment for if you don't want to read that comment up there: You can find Steven at the very top (highest floor) of Meteor cave.