Motivation. Getting into a battle is going to be counterproductive, but finding a motivating tool can be helpful; something that the child will find that will be beneficial to his life. If it is a matter of fear slowly desensitize the child by gradually making him less dependent on training wheels while you are right next to him, finally letting go.
Incentive ?? Kids are sometimes reluctant to do things because they fear them or just don't want to do something you suggest. Compliance is more likely with a carrot rather than a stick. Review or decide what the kid holds of value, activities, events, access, foods, & create an incentive program. If needed, divide the process into parts: getting on for 1 minute/ then 5/ then riding. Should pay off.