F.J.
I actually would stand behind my son when he had just turned 2, and physically move his feet properly then ask him to do it. The I would start one and he would push the other.. he got it pretty quickly .. of course its hard on your back!!!! You will be amazed once he starts going!!
Of course my other son doesn't even like to sit on the bike yet and he is now 3 1/2!! So if you have any ideas on that one let me know!!