I got him before the first gym. I think you just need to have your starter pokemon first.
SPOILER ALERT IF YOU CARE ENOUGH ABOUT WHAT THE BADGES DO AND DON'T WANT TO FIND OUT EARLY:
He wont listen till you get the first badge and thats only up to level 20. Obtained on level 15.
Actually, you can get Genesect before even starting the game, the delivery guy will be waiting for you in the Poke Center when you get there.
And yeah, he won't listen. Also worth mentioning, he levels up at a faster rate (since he appears to be a 'trade' pokemon) and he will obliterate almost anything you make him fight, regardless of whether he listens or not.