First time struggles!!!!

So me and my boyfriend are both virgins and have tried to have sex twice now but each time he takes his clothes off his boner disappears. Is this because of nerves or something? I’m worried I’m doing something wrong or it’s because of my body or something like that. Does anybody know why this is happening or have any tips for what I can do to make sure he stays hard?

Thank you ❤️❤️