I had one night stand with one guy, and now I am pregnant, but I want that this baby is from my boyfriend

So i had a one night enoucounter with some on Sept 27 maybe the 28th. He says he ejaculated im not positive he did. I dont recall my lmp as i am very irregular. however i tested positive for pregnancy oct 25 the doctor did a early ultrasound at the er oct 28th and saw nothing (this was a vaginal us) my beta counts then where 153. I the met with a obgyn the week of nov 19th where she did a early sonogram as well on nov 19th she said the baby messured 7 and a half weeks pregnant. They said i concived around the 10th of Oct. With a due date of july 2nd to the 5th (us tech said 5th.nurse says 2nd causs baby measures 3 days smaller then what they dr/nurse projected) them told me based of that my lmp was 9/24 . I dont want it to be the guy i had sex with that night of sep. Is it his or could ot be. Or is it my long term boyfriends baby. Seeing sperm lives 3 to 5 days im worried sick i want it to be my boyfriends