I'm pretty sure he's going to get bored with an electric ATV. They arent there yet as far as fun factor goes.
Just get a gas powered one
I started doing motorcross when I was 8, progressed to "adult" size bikes when I was 13. Starting racing ATV's when I was ~ 19. Theres nothing like the sound of a 2/4 stroke between your legs. You'll never reproduce that feeling with an electric motor.
My 2 cents. Good luck!