Having doubts on my obgyn at 6 months pregnant

Hi, I am on my 2nd pregnancy at 6months old now. And suddenly I have doubts on my obgyn because of some factors especially when he doesn't show the baby gender on the monitor rather than just saying it is 100%. he says that "the gender cannot be changed" i mean of coz, but why don't he show us the baby gender on the monitor so we as parents can confirm with what he say is real? I have enough struggles yet this doctor is not helping. What should i do? I have 3 more months to go.