well, you will never know unless you have a dna test when the baby is born., no one can tell you who's baby it is. If the outcome will matter to who will be bringing up the child, had you thought about abortion? You could have a medical abortion at this gestation. If abortion is unpalatable to you, then the only thing you can do is be honest to your bf. TBH until later, i would have thought ultrasound testing is rather unreliable for dates - they wont have even seen a HB