Definitely keep him around -- get him some catnip, make the couch his "spot," and post FOUND CAT signs in your building and on nearby lamp posts. If you don't hear from anyone in a week, consider Dante your new best friend.
He likely either jumped from another balcony (as others have guessed) or he may have hopped from the ground level from balcony to balcony and ended up in yours. He looks really sweet.
