I have the same kind of situation as your DS. I have never had contact with my biological father. My birth cert has a blank space next to father.

However, my Dad has been my dad since I was born. When I was 5 my surname was changed to my Dad's name via Deed Poll as my parents married.

I grew up believing dad was my real dad and was told the true story when I was 11.

To me, it means more that I have a different birth cert and my name was changed. Dad didn't have to do that, but he did because he wanted to be my Dad, IYKWIM?

Just another option to consider :-)