Give your son every opportunity for seat time!
Start your kids with the right foundation, consider trials.... CRF50 models are way too heavy for the l'il ones. Start them on a
Strider till they reach 5 y/o, then an
Oset 16.0 or
Kuberg Trials-E, then move them up to a proper trials bike when they're 12. You can be assured your child will probably out ride you if you follow this path and not follow the conventional route; and he will probably thank you when he outrides all his friends in the dirt. Here's an excellent thread on the pros & cons:
http://www.advrider.com/forums/showthread.php?t=847080
Explore the
Oset electric bike, the
Kuberg, or the
GasGas TXT-e . I don't recommend the
50cc Trials bikes for a beginner... very intimidating compared to CRF50/PW50/JR50.
The best thing about trials: tons of seat time in your backyard. and with the Oset, no noise. My kids can ride almost every day, and I don't need to load up for Carnegie, Hollister, etc. Being able to ride anytime anyday makes a huge difference in riding skills development and enjoying the sport.
Here's a list of local dealers if you're trying to source an Oset:
http://osetbikes.com/us/us-dealers/ or a Kuberg:
http://www.kuberg.com/Dealers.aspx . The GasGas is available from LewisposrtUSA:
http://www.lewisportusa.com/