How to find biological father's ethnicity?

[Observer]

You might try checking your original birth certificate (IF you are legally and adult). Since you were adopted by a step-father, it is quite that unless your were adopted in the same county you were born in your original birth certificate is still in the records of the county were you were born. However, I have located birth records that only indicated race. So if you are of European ancestry all you may find is that he was white. You might also find that that information is not correct.
My Grandmother "race" in my mothers birth records is Italian. My Grandfather was Italian-Swiss. My Grandmother was Irish/English.