Change negative self belief and become more confident

[Kay100]

I wonder which video does Leo talks about changing negative self beliefs and become more confident?

 

My son is 14 years old and our family life has always been great. But he had some problem in school. He has always had trouble with PE classes in school and that affected his self belief. He often says things like “I’m not strong enough to do that...” What is the best way to help him change his negative self belief?