@mommy2be,
Your doctor (who probably knows more than any of us) seems to be saying March 30, if I've got the math right. Which would mean your current boyfriend is the father. This is assuming that the measurements are correct and you're not carrying a much larger/smaller fetus than the norm.
BUT - you will not know
for certain unless you get a paternity test, which I believe you can only get after birth. If you wish to carry this baby to term, you need to be emotionally ready (as does your boyfriend) for the possibility that testing will prove that the child is not his. Does your ex know that you are pregnant, and that it might be his child? He needs to know as well for, if he is the father, he has financial obligations and parental rights in the matter.
I wish I could ease your mind.