Safety first. Insist upon a helmet each time your child is on the bike-with or without training wheels. Bikes that are too small are usually the easiest, when sitting on the seat the feet should easily reach the ground, with bent knees. On a small grassy incline, have your child lift the feet and allow the bike to slowly roll down the hill. Don't bother putting feet on the pedals until he's comfortable.
Show confidence. In them, start with trainers slowly go without while holding on to the seat. A quick walk progress slowly to a light jog let go then stay next to them and let them know what they achieved. Wear a helmet . This worked like a charm with my daughter.