When the major premise is wrong everything that comes after that is also wrong.
Who ever said parents have to hold the kids bike or that a kid needs training wheels? Both concepts just extend the amount of time that it takes a kid to learn to ride a bicycle.
1. Find a bike where you can lower the seat until the kid can put both feet on the ground.
2. While sitting on the saddle, let him push the bike with his feet on the ground. Gradually he'll learn to take giant steps, coasting more and pushing less. Slight sloping downhills are good.
3. From that point, lifting his feet and pedaling is a natural progression. There is no fear because he knows he can always reach the ground with his feet. In most cases the whole process only takes around 30 minutes. After a week or two you can begin to gradually raise the seat to a more efficient level.