Start by removing the pedals. Learning to ride a bike involves a chicken-and-egg problem: you need to learn to pedal in order to learn balance, but before you learn to balance you can’t practice pedaling. You can break these out by taking off the pedals so that he can straddle the bike easier and learn to balance by “scooting.” (These videos are in hd, to see the hd quality, click on the HD icon. For some reason I can’t get wordpress to embed the hd version directly.)
Once he has balance, learning to pedal is easy. Here is his first try (and second try) on the very next day.
Here is a previous installment. Next week, how to teach a 3 year old to swim.