Did you just tell him to stop wasting his time so he could waste his time even more?! And by playing Guitar Hero, you're setting yourself up to have to unlearn bad habits in order to have to learn how to play a (real) guitar, which most every teenager does eventually learn. In fact, I'm almost tempted to pick a guitar up and learn some basics some day, but as you know my instrument of choice makes louder sound and gets hit harder.
I'm pretty sure Hitmen has been playing guitar longer than he has Guitar Hero, and it probably doesn't greatly affect his playing ability. So shut up.
Exactly, he learned how to play guitar before he learned the game, so he's already got a solid base on playing guitar. So shut up.