I am so sorry for your loss. I don't understand at all why parents put that dilemma on a child. To me that is horrible to do to a grieving child, whether its grieving from a death of a parent or a divorce. You HAD a dad! HE will ALWAYS be your dad! You already HAVE a dad. He just has a new "address." I would never make my kids call someone else "dad." So to me, the only advice I have is to talk to your mom heart to heart without her boy toy around. Tell her you are still grieving your dad and you are not ready to call someone else dad yet. I hope this man is a good man and treats you just like his own. You are so young and there are so many things a little girl looks forward to having their dad do with them; walk down the aisle, daddy daughter dances, the scary evil eye toward your first boyfriend, just to name a few. You have been cut short those things. Who knows, maybe when it is time for you to say "I do" you may be able to really call him "dad." But YOUR dad will ALWAYS have a special place in your heart. Take care my friend. Once again, I am so sorry for your loss.
I just found out my dad passed away at 13. My mom just got a boyfriend and there now engaged, she wants me to call him dad and I dont want to what do I do?
Yowanna Cook's answer is a great one.
Tell your mother that you have a right to your feelings and that if and when you feel comfortable enough around this new man---and that he has gained your trust and love to the point that you want to call him "Dad"---you will do so.