Location: Meteor Falls

He ends up being in Meteor Falls in a new room after you beat the Elite Four.The Room is easy to access. You need Surf and Waterfall to get to it and you will see some ladders, follow them through and you'll get to a bit which has a ladder to the South West on a raised platform. This will take you outside the door you go in to the Steven Area. Follow it through and you'll see Steven. He will challenge you to a battle

Steven	Skarmory	Claydol	Aggron	Cradily	Armaldo	Metagross
Battle Type Items	Level 77	Level 75	Level 76	Level 76	Level 76	Level 78
	Attacks: Toxic Aerial Ace Spikes Steel Wing	Attacks: Reflect Light Screen Ancient Power Earthquake	Attacks: Thunder Earthquake Solar Beam Dragon Claw	Attacks: Giga Drain Ancient Power Ingrain Confuse Ray	Attacks: Water Pulse Ancient Power Aerial Ace Slash	Attacks: Earthquake Psychic Meteor Mash Shadow Ball
	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item	Hold Item: Sitrus Berry