Most of all, the confidence that comes from feeling that what you have to say is valued and listened to by your peers and family is profound. Chat together over a family meal every day. Leting your child see you learning is the most profound way of helping him learn to be flexible and open to ideas. Let your child see you modify your opinions, change your mind gracefully as a result of talking and finding out. They will come to appreciate that information changes as we encounter other people's ideas and opinions and that to make a change is not a sign of weakness but of strength and confidence.
Confidence comes from confident posture.
The way we sit or stand dictates the way we think and the way others react to us. Confident posture produces confident, positive thinking and people react to you positively. And that's the first step towards acting and reacting confidently to situations. So encourage your child to sit or stand tall, with forward shoulders, chest open, chin level, eyes looking ahead and slightly up. And of course active involvement in games and sports of any sort help develop a physical coordination and bearing that is confident.