How to refer to my birth father

Hi all. I am hoping for some advice. Thanks to a connection made on Ancestry.com, I have connected with my birth father, Lou, and his family 6 months ago. The new relationship is going very well. My adoptive parents passed away a couple of years ago from illness. I was especially close with them. I refer to Lou as my birth father when mentioning him to friends, but am realizing that it may not be the most elegant way to refer to him when introducing him to people in person. I cannot bring myself to calling him dad, or saying that he's my father. I had a father for 46 years, and am not comfortable with handing the dad title over, despite Lou being a wonderful and loving presence in my life. Also, all of my friends and most acquaintances of mine know that I was especially close with my adoptive family; it'd be awkward to then introduce Lou with "this is my father", etc. Have any of you encountered the same scenario? I know Lou wants to be 'dad' to me, not 'birth father'. Is it something he should try to get accustomed to, or is there another approach I can take when introducing him? Thanks in advance for your thoughts.