Respuesta :
Incredulous means suprised or that you find something unbelieveable. In this context a good synonym would be "shocked" or "stunned."
"Incredulous" means that a person is skeptical about something, or that he finds something hard to believe. In this case, the character is incredulous about his ability to hold his breath without strain for two minutes. This means that he finds this fact difficult to believe, as it is so amazing to him.