No way to know for sure without a DNA test.
OK, now you are saying that not only did you have sex with your boyfriend up until the 9th, but also on the 14th? Is that what you meant when you said just above in the "Comments" section that "my boyfriend and i had a contact also on the same day nov 14 he accidentally inseminate inside me"? If you had sex with both guys on the 14th and both guys ejaculated, there is really no way to know at this point who is the father (unless you take a very costly pre-natal DNA test from Ravgen or the DDC. If you wait and do a DNA test after the baby comes, the price drops to about a tenth of the cost of pre-natal testing.)
I don't think it sounds like your boyfriend is the most likely father, I think it sounds like the other guy is the most likely father.
What kind of position are you in, did you tell your boyfriend that he is the father?
Did they give you a due date of August 8 when they did that sixth-week ultrasound? If so, and by the count of 6w5d on December 18, it sounds like you conceived around November 15. Are your menstrual months long or irregular? (Because this conception date is a ways from your last period, but that can readily happen when a woman has irregular cycles.) I'm sorry, but what you are saying from the ultrasound puts the other guy in the most likely position to be the dad.