Do you have a due date given to you from an ultrasound in your 6th or 7th week of pregnancy? Without that, I don't think you will be able to be sure of anything until you have a DNA test.
And has the baby been born? If so, have you had the chance to see any baby pictures of your boyfriend when he was an infant, and do they compare? (I'm talking about comparing a photo of the new baby to photo of boyfriend at same age, not comparing the actual wiggling and kicking new baby to an old photo of boyfriend. Sometimes, when compared photo to photo, the two images could be of the same baby.)
Anyway, leaving aside that unscientific method for a moment, what I was wanting to know is when you first were told the due date of March 6, what point was that in your pregnancy? 6th week? 7th week? Or was it later?