I started when I was 5 years old and I consider myself a normal human being.
I think the more appropiate answer is not when he should be starting to game but what kind of games he is going to play.
Edit: It's also important how much time he is going to spend playing videogames!
"I've Underestimated the Horse Power from Mario Kart 8, I'll Never Doubt the WiiU's Engine Again"