We bought our son at 9yrs (he's 10 now) the Honda CRF 70.
The saddle is low enough for him to flat foot standing or sitting. The CRF 70 shifts without a clutch... hence, virtually no stalling. My intent was to have him get used to the sport in all aspects and make riding the bike as close to riding his mountain bike as possible...
We've had the Honda for almost 7 months and he still has fun on it. We may upgrade at the end of the summer...the little 70 will stay in the stable and be used by younger cousins and baby brother in the future.
We Paid much less than $1700 new ... The dealers are blowing out '06 models in my area-perhaps yours too.
Get your son all the safety equipment: Helmet,goggles, chest protector, gloves and boots. Hindsight being 20/20, I would have started out with knee protection as well.
Have fun with your boy... this is a great part of being a dad IMHO.
Ken