Really need to decide what babies last name will be!

I'm still with his father. We're not married, we're leaning towards giving him my last name. The main reason being that my fiance and his father are not on good terms at all. He completely disowned him when he moved in with me and then again when he found out I was pregnant. He told him when we get married my fiance better take my last name (and my families more than welcoming to him). I just feel backwards about it though and I know my fiance hates the situation all together so its hard to talk about.