He will give you one of the three Hoenn starters, and will also notify you of Latios' (Or LATIAS) arrival to the region.
Steven isn't battleable in the game.
You can't battle Steven in Pokemon SoulSilver or HeartGold.
In Saffron City
you cant beat steven in pokemon soul silver only in emerald,ruby and sapphire
steven has to be found in the museoum in peweter city an trade pokemon with him at saffron city
to find meteorites and crystals.
Pewter museum!! :D
Unfortunately, you cannot battle Steven on Pokemon HeartGold or SoulSilver unless an event comes in the future.
He is in Silph Co. but you need to trade rotom over from Pokemon platinum
You can get latios after you meet Steven, and latias through the enigma stone event.
You don't find Steven in Mount Silver. You find Steven in Silph Co. after beating Red. He will give you a Hoenn starter.
After you get your Hoenn starter and speak to Steven in the Pewter Museum, Steven will be in the Silph Co. bottom floor. He has a Beldum with a Dawn Stone to trade for a Forretress.