Guess the first thing to learn is balance. Start with a bike that fits and that the saddle can be lowered enough so when sitting, his feet can touch the ground. Remove the pedals. Now you can get him started on a level or slight downhill surface (I'm thinking rim brakes, not coaster since pedals are removed). He can then coast and put his feet down until he gets balancing down and won't worry about falling over, while his confidence builds that he can ride without touching down. Then reattach the pedals and off he goes!