After my son got very good on a scooter, I suggested he try gliding down a very slight incline (our driveway) on a bike that was too small for him to pedal. This allowed him to easily put both of his feet flat on the ground for balance as needed. When you go down a slight hill, you get the speed that's necessary to keep the bike upright.
Then I encouraged him to try pedaling a bit. From there, he learned how to ride a bike.
When he first started, I put the seat a bit low so that again, he could easily put his feet on the ground for balance. I did this with my daughter too. If they can't put their feet flat, they're going to fall eventually (my kids anyway).
I always had him position the pedals in the optimal position for starting, too, and ALWAYS reminded him to LOOK AHEAD, NOT DOWN!!
Knee pads and other pads (+ helmet)are your best friends in this situation. They reduce injury and enhance courage.
HTH.
