You have to take your lead from him. Our children know better than we do what they can and can’t do. There are individual differences among children in their willingness to take risks. Chances are your son will continue to be somewhat cautious no matter what you do to help him get over this tendency.
But you also need to check your own behavior and determine whether you are making him more fearful by pushing him to do things he is afraid to try. Most children at 5½ can’t ride a bike without training wheels, so there is nothing unusual about this fear. He is more likely to carry his fears forward if you shame him about things he doesn’t want to try. I don’t mean to imply that you shouldn’t encourage him to tackle ever more complex activities, for gentle pacing from parents is extremely important in helping children move up the developmental ladder.
Your major allies will be his friends. As he sees more and more of them riding two-wheelers, he will want to do likewise. Praise him when he attempts things you want to encourage, but try to be casual about refusals or accidents. Say something reassuring, like, “You’ll be able to do that before long.”
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.