Steven is most likely referring to a character that showed up in the Pokemon Mega Evolution Special II named Alain. In the episode he fights Steven's Mega Metagross with his Mega Charizard X(the black Charizard). Both of them encounter Mega Rayquaza later on in the episode. You can find more information here in Bulbapedia and learn his story a little more in-depth but dow here is a small little summary of what he mainly does in these episodes.
Alain (Japanese: アラン Alain) is a recurring character in the Pokémon anime. He is the main protagonist of the side story episodes Mega Evolution Special I and SS031 (Special II). Alain is a Pokémon Trainer seeking to defeat all Mega Evolved Pokémon using his Mega Charizard X. -Bulbapedia
Hope this helped.
Source here <-Basically the link to Bulbapedia