Changing your last name?

My fiancé has 2 dads. But he only considers his step father as his father. His step father has been there since he was a young child and really raised him and supported him with everything. His bio dad was kind of a piece of garbage to everyone and hasn’t been in contact with my fiancé or siblings.
We spoke about it between us and the family and he is changing his name!! So now we are both changing our name to something new and it feels more special than it would otherwise. It’s a big deal and I’m proud he was able to make the decision.
Future Mr and Mrs B!
instead of “F”