Is your scan supposed to measure up until the day you had sex

When you take off the two weeks they add onto your GA is that supposed to be the day you conceived ? My boyfriend keeps pulling my hair and hurting me saying the baby is not his because the dates are not adding up for him, I can’t leave because I love him and I have nowhere else to go, I’m 17 Turning 18 and I honestly don’t know much about pregnancy just the birth control side of things. I’m trying to explain to him that they add two weeks on and he’s just not listening he’s 17 too by the way