Pokémon Omega Ruby and Pokémon Alpha Sapphire announced

Didn't CoroCoro says he was learning about Mega Evolution? The Kanto starters+their mega stones were our first real megas(that Lucario doesn't count), could make sense to be his first while learning about megas.

As a nod to your trainer in X or Y being given a Kanto starter to run the same task? Yeah.

But if you're talking about the actual first mega evolution, it's heavily implied (if not directly stated) that a Lucario led to the first mega evolution.