Hello. Hoping someone can answer this question. My grandfather was

I had the same story and my dad was considered American   On his immigration papers its states date of citizenship which is listed as his date of birth.   When I obtained my dual citizenship through my mother I was told that my dad "qualified" for Italian citizenship if he wanted.   At that point my dad passed away and would have been 90 years old.   He came here on an Italian passport before he turned 21 in 1947 so yes its confusing but answering your question I believe your grandfather was considered American if he came to the USA before he was 21 years old

it would be questionable if your grandfather's father, in taking American citizenship, renounced the Italian one - this is because in Italy, the surname, in the years in which your grandfather was born, was transmitted from father to son and with it the citizenship. The fact that his mother was an Italian citizen has no influence on the dynastic line.