Hi OP, I feel for you it must be a difficult position to be in. If I was not married and he had already broken an engagement previously, I would give the baby my last name. There is nothing to stop you naming him as the father on the birth certificate but naming the baby with your last name. If you were married I think it's completely different. It would be very difficult for me to give the baby the dad's name - if it doesn't work out in the long run, they will always have that name. If it all does work out, it is actually not difficult to change the name legally, it is a simple form.