First, good on you for looking for neck protection for the lad and being willing to spend the $$ to protect him. To the best of my knowledge, there's not a secret-best-for-cheap neck brace out there... like most things, especially those that claim to protect, you get what you pay for. This is no piece of gear to look to china (or other knock-off, out of the reach of litigation) for. Riding and racing motocross/off-road can be dangerous (and expensive). We can reduce the potential injury with the right gear and a lot of practice. Neither are cheap.
That being said, I'd default to the innovator, Leatt.
Best of luck and let us know how his career goes!