Go on the Family forum. There is a very exhaustive thread on the subject. General rule is:
1. Take off training wheels (greatest cycling misnomer IMHO) and pedals (remembering LH pedal has LH thread) and the bike needs to be small enough for him to get his feet comfortably on ground
2. find slight, preferably grassy slope leading to flat ground
3. let him roll down slope - he will soon get hang of keeping his feet off ground and will roll some distance balancing
4. when he is confident let him start from higher up slope
5. when he can go some distance under control. Put pedals back on
6. if 5 successful it will be your turn to worry.

The reasoning behind taking off the pedals is that it reduces the things he has to do simultaneously, i.e. he only has to worry about balancing and steering. When he's got the hang of that, then he can learn to pedal. A bit like learning to drive, when you wonder how people manage to let clutch in change gear, look in mirror, steer and signal, more or less all at once. Assuming that you didn't learn to drive on an automatic box, that is.