I'm in that situation. DH has been around since DS was 2. His dad sees him twice a month. He asked us the other day I he could call DH dad. He says he has 2 daddies and that DH is his favorite daddy. We have NEVER pushed anything on him. I actually had him calling him by his name. But after DH and I had our 2nd LO together the oldest said he didn't like his brother and sister not saying daddy. So he says he wants to call him dad too. (He's almost 6 now)
My daughter is three. Her dad is in the picture but he wasn't so much in the beginning. My son's dad was much more in it than anyone. She decided to call my ex "daddy" and being spanish speakers, her biological dad is "papi". There isn't much explaining I could do for a three year old.
Even though my sons father wasn't in the picture. I didn't allow my son to call my husband Dad in the beginning. He was 10 when we got married. He asked me a couple months before our wedding if he can call him Dad. I told him that was completely up to him. As soon as that ring was on my finger it has been Dad since.
I don't believe before you are married to a man that they should call them Dad. But let them decide what they are comfortable with calling their Step Dads. But my sons father has never and will never be in the picture. So my son was wanting to call the man that is raising him Dad.